匿名函数

匿名函数（Anonymous functions），也叫闭包函数（closures），允许临时创建一个没有指定名称的函数。最经常用作回调函数（callback）参数的值。当然，也有其它应用的情况。

匿名函数目前是通过Closure类来实现的。

Example #1 匿名函数示例

<?php
echo preg_replace_callback('~-([a-z])~', function ($match) {
    return strtoupper($match[1]);
}, 'hello-world');
// 输出 helloWorld
?>

闭包函数也可以作为变量的值来使用。PHP 会自动把此种表达式转换成内置类Closure的对象实例。把一个 closure 对象赋值给一个变量的方式与普通变量赋值的语法是一样的，最后也要加上分号：

Example #2 匿名函数变量赋值示例

<?php
$greet = function($name)
{
    printf("Hello %s\r\n", $name);
};
$greet('World');
$greet('PHP');
?>

闭包可以从父作用域中继承变量。任何此类变量都应该用use语言结构传递进去。 PHP 7.1 起，不能传入此类变量：superglobals、$this或者和参数重名。

Example #3 从父作用域继承变量

<?php
$message = 'hello';
// 没有 "use"
$example = function () {
    var_dump($message);
};
echo $example();
// 继承 $message
$example = function () use ($message) {
    var_dump($message);
};
echo $example();
// Inherited variable's value is from when the function
// is defined, not when called
$message = 'world';
echo $example();
// Reset message
$message = 'hello';
// Inherit by-reference
$example = function () use (&$message) {
    var_dump($message);
};
echo $example();
// The changed value in the parent scope
// is reflected inside the function call
$message = 'world';
echo $example();
// Closures can also accept regular arguments
$example = function ($arg) use ($message) {
    var_dump($arg . ' ' . $message);
};
$example("hello");
?>

以上例程的输出类似于：

Notice: Undefined variable: message in /example.php on line 6
NULL
string(5) "hello"
string(5) "hello"
string(5) "hello"
string(5) "world"
string(11) "hello world"

这些变量都必须在函数或类的头部声明。从父作用域中继承变量与使用全局变量是不同的。全局变量存在于一个全局的范围，无论当前在执行的是哪个函数。而闭包的父作用域是定义该闭包的函数（不一定是调用它的函数）。示例如下：

Example #4 Closures 和作用域

<?php
// 一个基本的购物车，包括一些已经添加的商品和每种商品的数量。
// 其中有一个方法用来计算购物车中所有商品的总价格，该方法使
// 用了一个 closure 作为回调函数。
class Cart
{
    const PRICE_BUTTER  = 1.00;
    const PRICE_MILK    = 3.00;
    const PRICE_EGGS    = 6.95;
    protected   $products = array();
    
    public function add($product, $quantity)
    {
        $this->products[$product] = $quantity;
    }
    
    public function getQuantity($product)
    {
        return isset($this->products[$product]) ? $this->products[$product] :
               FALSE;
    }
    
    public function getTotal($tax)
    {
        $total = 0.00;
        
        $callback =
            function ($quantity, $product) use ($tax, &$total)
            {
                $pricePerItem = constant(__CLASS__ . "::PRICE_" .
                    strtoupper($product));
                $total += ($pricePerItem * $quantity) * ($tax + 1.0);
            };
        
        array_walk($this->products, $callback);
        return round($total, 2);;
    }
}
$my_cart = new Cart;
// 往购物车里添加条目
$my_cart->add('butter', 1);
$my_cart->add('milk', 3);
$my_cart->add('eggs', 6);
// 打出出总价格，其中有 5% 的销售税.
print $my_cart->getTotal(0.05) . "\n";
// 最后结果是 54.29
?>

Example #5 Automatic binding of$this

<?php
class Test
{
    public function testing()
    {
        return function() {
            var_dump($this);
        };
    }
}
$object = new Test;
$function = $object->testing();
$function();
    
?>

以上例程会输出：

object(Test)#1 (0) {
}

以上例程在PHP 5.3中的输出：

Notice: Undefined variable: this in script.php on line 8
NULL

As of PHP 5.4.0, when declared in the context of a class, the current class is automatically bound to it, making$thisavailable inside of the function's scope. If this automatic binding of the current class is not wanted, then static anonymous functions may be used instead.

Static anonymous functions

As of PHP 5.4, anonymous functions may be declared statically. This prevents them from having the current class automatically bound to them. Objects may also not be bound to them at runtime.

Example #6 Attempting to use$thisinside a static anonymous function

<?php
class Foo
{
    function __construct()
    {
        $func = static function() {
            var_dump($this);
        };
        $func();
    }
};
new Foo();
?>

以上例程会输出：

Notice: Undefined variable: this in %s on line %d
NULL

Example #7 Attempting to bind an object to a static anonymous function

<?php
$func = static function() {
    // function body
};
$func = $func->bindTo(new StdClass);
$func();
?>

以上例程会输出：

Warning: Cannot bind an instance to a static closure in %s on line %d

更新日志

版本	说明
7.1.0	Anonymous functions may not close over superglobals,`$this`, or any variable with the same name as a parameter.
5.4.0	`$this`可用于匿名函数。
5.3.0	可以使用匿名函数。

注释

Note:可以在闭包中使用func_num_args()，func_get_arg()和func_get_args()。

Watch out when 'importing' variables to a closure's scope -- it's easy to miss / forget that they are actually being *copied* into the closure's scope, rather than just being made available.
So you will need to explicitly pass them in by reference if your closure cares about their contents over time:
<?php
$result = 0;
$one = function()
{ var_dump($result); };
$two = function() use ($result)
{ var_dump($result); };
$three = function() use (&$result)
{ var_dump($result); };
$result++;
$one();  // outputs NULL: $result is not in scope
$two();  // outputs int(0): $result was copied
$three();  // outputs int(1)
?>
Another less trivial example with objects (what I actually tripped up on):
<?php
//set up variable in advance
$myInstance = null;
$broken = function() uses ($myInstance)
{
  if(!empty($myInstance)) $myInstance->doSomething();
};
$working = function() uses (&$myInstance)
{
  if(!empty($myInstance)) $myInstance->doSomething();
}
//$myInstance might be instantiated, might not be
if(SomeBusinessLogic::worked() == true)
{
  $myInstance = new myClass();
}
$broken();  // will never do anything: $myInstance will ALWAYS be null inside this closure.
$working();  // will call doSomething if $myInstance is instantiated
?>

Every instance of a lambda has own instance of static variables. This provides for great event handlers, accumulators, etc., etc.
Creating new lambda with function() { ... }; expression creates new instance of its static variables. Assigning a lambda to a variable does not create a new instance. A lambda is object of class Closure, and assigning lambdas to variables has the same semantics as assigning object instance to variables.
Example script: $a and $b have separate instances of static variables, thus produce different output. However $b and $c share their instance of static variables - because $c is refers to the same object of class Closure as $b - thus produce the same output.
#!/usr/bin/env php
<?php
function generate_lambda() : Closure
{
    # creates new instance of lambda
  return function($v = null) {
    static $stored;
    if ($v !== null)
      $stored = $v;
    return $stored;
  };
}
$a = generate_lambda(); # creates new instance of statics
$b = generate_lambda(); # creates new instance of statics
$c = $b;                 # uses the same instance of statics as $b
$a('test AAA');
$b('test BBB');
$c('test CCC'); # this overwrites content held by $b, because it refers to the same object
var_dump([ $a(), $b(), $c() ]);
?>
This test script outputs:
array(3) {
 [0]=>
 string(8) "test AAA"
 [1]=>
 string(8) "test CCC"
 [2]=>
 string(8) "test CCC"
}

To recursively call a closure, use this code.
<?php
$recursive = function () use (&$recursive){
  // The function is now available as $recursive
}
?>
This DOES NOT WORK
<?php
$recursive = function () use ($recursive){
  // The function is now available as $recursive
}
?>

You may have been disapointed if you tried to call a closure stored in an instance variable as you would regularly do with methods:
<?php
$obj = new StdClass();
$obj->func = function(){
 echo "hello";
};
//$obj->func(); // doesn't work! php tries to match an instance method called "func" that is not defined in the original class' signature
// you have to do this instead:
$func = $obj->func;
$func();
// or:
call_user_func($obj->func);
// however, you might wanna check this out:
$array['func'] = function(){
 echo "hello";
};
$array['func'](); // it works! i discovered that just recently ;)
?>
Now, coming back to the problem of assigning functions/methods "on the fly" to an object and being able to call them as if they were regular methods, you could trick php with this lawbreaker-code:
<?php
class test{
 private $functions = array();
 private $vars = array();
 
 function __set($name,$data)
 {
 if(is_callable($data))
  $this->functions[$name] = $data;
 else
  $this->vars[$name] = $data;
 }
 
 function __get($name)
 {
 if(isset($this->vars[$name]))
  return $this->vars[$name];
 }
 
 function __call($method,$args)
 {
 if(isset($this->functions[$method]))
 {
  call_user_func_array($this->functions[$method],$args);
 } else {
  // error out
 }
 }
}
// LET'S BREAK SOME LAW NOW!
$obj = new test;
$obj->sayHelloWithMyName = function($name){
 echo "Hello $name!";
};
$obj->sayHelloWithMyName('Fabio'); // Hello Fabio!
// THE OLD WAY (NON-CLOSURE) ALSO WORKS:
function sayHello()
{
 echo "Hello!";
}
$obj->justSayHello = 'sayHello';
$obj->justSayHello(); // Hello!
?>
NOTICE: of course this is very bad practice since you cannot refere to protected or private fields/methods inside these pseudo "methods" as they are not instance methods at all but rather ordinary functions/closures assigned to the object's instance variables "on the fly". But I hope you've enjoyed the jurney ;)

In case you were wondering (cause i was), anonymous functions can return references just like named functions can. Simply use the & the same way you would for a named function...right after the `function` keyword (and right before the nonexistent name).
<?php
  $value = 0;
  $fn = function &() use (&$value) { return $value; };
  $x =& $fn();
  var_dump($x, $value);    // 'int(0)', 'int(0)'
  ++$x;
  var_dump($x, $value);    // 'int(1)', 'int(1)'

<?php
  /*
  (string) $name Name of the function that you will add to class.
  Usage : $Foo->add(function(){},$name);
  This will add a public function in Foo Class.
  */
  class Foo
  {
    public function add($func,$name)
    {
      $this->{$name} = $func;
    }
    public function __call($func,$arguments){
      call_user_func_array($this->{$func}, $arguments); 
    }
  }
  $Foo = new Foo();
  $Foo->add(function(){
    echo "Hello World";
  },"helloWorldFunction");
  $Foo->add(function($parameterone){
    echo $parameterone;
  },"exampleFunction");
  $Foo->helloWorldFunction(); /*Output : Hello World*/
  $Foo->exampleFunction("Hello PHP"); /*Output : Hello PHP*/
?>

PERFORMANCE BENCHMARK 2017!
I decided to compare a single, saved closure against constantly creating the same anonymous closure on every loop iteration. And I tried 10 million loop iterations, in PHP 7.0.14 from Dec 2016. Result:
a single saved closure kept in a variable and re-used (10000000 iterations): 1.3874590396881 seconds
new anonymous closure created each time (10000000 iterations): 2.8460240364075 seconds
In other words, over the course of 10 million iterations, creating the closure again during every iteration only added a total of "1.459 seconds" to the runtime. So that means that every creation of a new anonymous closure takes about 146 nanoseconds on my 7 years old dual-core laptop. I guess PHP keeps a cached "template" for the anonymous function and therefore doesn't need much time to create a new instance of the closure!
So you do NOT have to worry about constantly re-creating your anonymous closures over and over again in tight loops! At least not as of PHP 7! There is absolutely NO need to save an instance in a variable and re-use it. And not being restricted by that is a great thing, because it means you can feel free to use anonymous functions exactly where they matter, as opposed to defining them somewhere else in the code. :-)

As of PHP 7.0, you can use IIFE(Immediately-invoked function expression) by wrapping your anonymous function with ().
<?php
$type = 'number';
var_dump( ...( function() use ($type) { 
  if ($type=='number') return [1,2,3]; 
  else if ($type=='alphabet') return ['a','b','c'];
} )() );
?>

When using anonymous functions as properties in Classes, note that there are three name scopes: one for constants, one for properties and one for methods. That means, you can use the same name for a constant, for a property and for a method at a time.
Since a property can be also an anonymous function as of PHP 5.3.0, an oddity arises when they share the same name, not meaning that there would be any conflict.
Consider the following example:
<?php
  class MyClass {
    const member = 1;
    
    public $member;
    
    public function member () {
      return "method 'member'";
    }
    
    public function __construct () {
      $this->member = function () {
        return "anonymous function 'member'";
      };
    }
  }
  
  header("Content-Type: text/plain");
  
  $myObj = new MyClass();
  var_dump(MyClass::member); // int(1)
  var_dump($myObj->member);  // object(Closure)#2 (0) {}
  var_dump($myObj->member()); // string(15) "method 'member'"
  $myMember = $myObj->member;
  var_dump($myMember());   // string(27) "anonymous function 'member'"
?>
That means, regular method invocations work like expected and like before. The anonymous function instead, must be retrieved into a variable first (just like a property) and can only then be invoked.
Best regards,

If you want to create and then immediately call a closure directly, in-line, and immediately get its return value (instead of the closure reference itself), then the proper syntax is as follows:
<?php
$a = 'foo'; $b = 'bar';
$test = (function() use($a,$b) { return $a . $b; })();
echo $test;
?>
As for why you would want to do that? Well, that's up to you. I'm sure there are some legitimate reasons. It's a pretty common pattern in some other famous scripting languages. But if you're doing this in PHP, you should think carefully and ask yourself if you really have a good reason for it, or if you should just go and re-structure your code instead. ;-)

Some hosts are installing the eAccelerator opcode cache with PHP 5.4, and current production versions of this opcode cache break closures. If you find that your nice, working closures break when you load your code to a hosted website, check for eAccelerator (e.g. by calling phpinfo() and checking the output).
Simple fixes include:
* disabling eAccelerator and opcode caching
* replacing eAccelerator with Zend's opcache
* reverting to PHP 5.3
Apparently, the eAccelerator project has solved this issue, but hosts move slowly so I recommend removing eAccelerator from the equation for now.

As an alternative to gabriel's recursive construction, you may instead assign the recursive function to a variable, and use it by reference, thus:
<?php
$fib = function($n)use(&$fib)
{
  if($n == 0 || $n == 1) return 1;
  return $fib($n - 1) + $fib($n - 2);
};
echo $fib(10);
?>
Hardly a sensible implementation of the Fibonacci sequence, but that's not the point! The point is that the variable needs to be used by reference, not value.
Without the '&', the anonymous function gets the value of $fib at the time the function is being created. But until the function has been created, $fib can't have it as a value! It's not until AFTER the function has been assigned to $fib that $fib can be used to call the function - but by then it's too late to pass its value to the function being created!
Using a reference resolves the dilemma: when called, the anonymous function will use $fib's current value, which will be the anonymous function itself.
At least, it will be if you don't reassign $fib to anything else between creating the function and calling it:
<?php
$fib = function($n)use(&$fib)
{
  if($n == 0 || $n == 1) return 1;
  return $fib($n - 1) + $fib($n - 2);
};
$lie = $fib;
$fib = function($n)
{
  return 100;
};
echo $lie(10); // 200, because $fib(10 - 1) and $fib(10 - 2) both return 100.
?>
Of course, that's true of any variable: if you don't want its value to change, don't change its value.
All the usual scoping rules for variables still apply: a local variable in a function is a different variable from another one with the same name in another function:
<?php
$fib = function($n)use(&$fib)
{
  if($n == 0 || $n == 1) return 1;
  return $fib($n - 1) + $fib($n - 2);
};
$bark = function($f)
{
  $fib = 'cake';  // A totally different variable from the $fib above.
  return 2 * $f(5);
};
echo $bark($fib); // 16, twice the fifth Fibonacci number
?>

If you want to make sure that one of the parameters of your function is a Closure, you can use Type Hinting.
see: http://php.net/manual/en/language.oop5.typehinting.php
Example:
<?php
class TheRoot
{
  public function poidh($param) {
    echo "TheRoot $param!";
  }  
}
class Internet
{
  # here, $my_closure must be of type object Closure
  public function run_my_closure($bar, Closure $my_closure) {
    $my_closure($bar);
  }  
}
$Internet = new Internet();
$Root = new TheRoot();
$Internet->run_my_closure($Root, function($Object) {
  $Object->poidh(42);
});
?>
The above code simply yields:
"TheRoot 42!"
NOTE: If you are using namespaces, make sure you give a fully qualified namespace.
print_r() of Internet::run_my_closure's $my_closure
<?php
Closure Object
(
  [parameter] => Array
    (
      [$Object] => 
    )
)
?>
var_dump() of Internet::run_my_closure's $my_closure
<?php
object(Closure)#3 (1) {
 ["parameter"]=>
 array(1) {
  ["$Object"]=>
  string(10) ""
 }
}
?>

Will result in a "Parse error: syntax error, unexpected '[', expecting ',' or ')' ... "
<?php
$fruits = ['apples', 'oranges'];
$example = function () use ($fruits[0]) {
  echo $fruits[0]; 
};
$example();
?>
Would have to do this:
<?php
$fruits = ['apples', 'oranges'];
$example = function () use ($fruits) {
  echo $fruits[0]; // will echo 'apples'
};
$example();
?>
Or this instead:
<?php
$fruits = ['apples', 'oranges'];
$fruit = $fruits[0];
$example = function () use ($fruit) {
  echo $fruit; // will echo 'apples'
};
$example();

You can always call protected members using the __call() method - similar to how you hack around this in Ruby using send.
<?php
class Fun
{
 protected function debug($message)
 {
  echo "DEBUG: $message\n";
 }
 public function yield_something($callback)
 {
  return $callback("Soemthing!!");
 }
 public function having_fun()
 {
  $self =& $this;
  return $this->yield_something(function($data) use (&$self)
  {
   $self->debug("Doing stuff to the data");
   // do something with $data
   $self->debug("Finished doing stuff with the data.");
  });
 }
 // Ah-Ha!
 public function __call($method, $args = array())
 {
  if(is_callable(array($this, $method)))
   return call_user_func_array(array($this, $method), $args);
 }
}
$fun = new Fun();
echo $fun->having_fun();
?>

A common way to avoid contaminating Javascript global space with unneeded variables is to move the code into an immediately called anonymous closure.
(function(){ ... })()
The equivalent way to do that in PHP 5.3+ is
call_user_func(function() use(closure-vars){ ... });

Here is a simple example to use clousers
<?php 
function add($x,$y){
  return $x+$y();  
}
echo add(3,function(){
  return 5;
});
?>
prints 8

If you want to make a recursive closure, you will need to write this:
$some_var1="1";
$some_var2="2";
function($param1, $param2) use ($some_var1, $some_var2)
{
//some code here
call_user_func(__FUNCTION__, $other_param1, $other_param2);
//some code here
}
If you need to pass values by reference you should check out
http://www.php.net/manual/en/function.call-user-func.php
http://www.php.net/manual/en/function.call-user-func-array.php
If you're wondering if $some_var1 and $some_var2 are still visible by using the call_user_func, yes, they are available.

Supported by PHP 7.0+ only.
<?php
  (function($name){
    echo 'My name is ' . $name;
  })('Wu Xiancheng');
?>

Anonymous functions are great for events!
<?php
class Event {
 public static $events = array();
 
 public static function bind($event, $callback, $obj = null) {
  if (!self::$events[$event]) {
   self::$events[$event] = array();
  }
  
  self::$events[$event][] = ($obj === null) ? $callback : array($obj, $callback);
 }
 
 public static function run($event) {
  if (!self::$events[$event]) return;
  
  foreach (self::$events[$event] as $callback) {
   if (call_user_func($callback) === false) break;
  }
 }
}
function hello() {
 echo "Hello from function hello()\n";
}
class Foo {
 function hello() {
  echo "Hello from foo->hello()\n";
 }
}
class Bar {
 function hello() {
  echo "Hello from Bar::hello()\n";
 }
}
$foo = new Foo();
// bind a global function to the 'test' event
Event::bind("test", "hello");
// bind an anonymous function
Event::bind("test", function() { echo "Hello from anonymous function\n"; });
// bind an class function on an instance
Event::bind("test", "hello", $foo);
// bind a static class function
Event::bind("test", "Bar::hello");
Event::run("test");
/* Output
Hello from function hello()
Hello from anonymous function
Hello from foo->hello()
Hello from Bar::hello()
*/
?>

Using closure to encapsulate environment
<?php
  $fib = function($n) use(&$fib) {
    if($n == 0 || $n == 1) return 1;
    return $fib($n - 1) + $fib($n - 2);
  };
  echo $fib(2) . "\n"; // 2
  $lie = $fib;
  $fib = function(){die('error');};//rewrite $fib variable 
  echo $lie(5); // error  because $fib is referenced by closure
?>
Alternative Fibonacci implementation using a self called function like javascript to encapsulate references variables.
<?php
$fib = call_user_func(function(){
  
  $fib = function($n) use(&$fib) {
    if($n == 0 || $n == 1) return 1;
    return $fib($n - 1) + $fib($n - 2);
  };
  return $fib;
});
echo $fib(2) . "\n";//2
$ok = $fib;
$fib = function(){die('error')};//rewrite $fib variable but don't referenced $fib used by closure
echo $ok(5);//result ok 
?>

<?php
/* 
 * An example showing how to use closures to implement a Python-like decorator 
 * pattern.
 *
 * My goal was that you should be able to decorate a function with any
 * other function, then call the decorated function directly: 
 *
 * Define function:     $foo = function($a, $b, $c, ...) {...}
 * Define decorator:    $decorator = function($func) {...}
 * Decorate it:       $foo = $decorator($foo)
 * Call it:         $foo($a, $b, $c, ...)
 *
 * This example show an authentication decorator for a service, using a simple
 * mock session and mock service. 
 */
 
session_start();
/* 
 * Define an example decorator. A decorator function should take the form:
 * $decorator = function($func) {
 *   return function() use $func) {
 *     // Do something, then call the decorated function when needed:
 *     $args = func_get_args($func);
 *     call_user_func_array($func, $args);
 *     // Do something else.
 *   };
 * };
 */
$authorise = function($func) {
  return function() use ($func) {
    if ($_SESSION['is_authorised'] == true) {
      $args = func_get_args($func);
      call_user_func_array($func, $args);
    }
    else {
      echo "Access Denied";
    }
  };
};
/* 
 * Define a function to be decorated, in this example a mock service that
 * need to be authorised. 
 */ 
$service = function($foo) {
  echo "Service returns: $foo";
};
/* 
 * Decorate it. Ensure you replace the origin function reference with the
 * decorated function; ie just $authorise($service) won't work, so do
 * $service = $authorise($service)
 */
$service = $authorise($service);
/* 
 * Establish mock authorisation, call the service; should get 
 * 'Service returns: test 1'. 
 */
$_SESSION['is_authorised'] = true;
$service('test 1');
/* 
 * Remove mock authorisation, call the service; should get 'Access Denied'. 
 */
$_SESSION['is_authorised'] = false;
$service('test 2');
?>

Here is an example of one way to define, then use the variable ( $this ) in Closure functions. The code below explores all uses, and shows restrictions.
The most useful tool in this snippet is the requesting_class() function that will tell you which class is responsible for executing the current Closure(). 
Overview:
-----------------------
Successfully find calling object reference.
Successfully call $this(__invoke);
Successfully reference $$this->name;
Successfully call call_user_func(array($this, 'method'))
Failure: reference anything through $this->
Failure: $this->name = ''; 
Failure: $this->delfect(); 
<?php
 
  
  
  function requesting_class()
  {
    foreach(debug_backtrace(true) as $stack){
      if(isset($stack['object'])){
        return $stack['object'];
      }
    }
    
  }
  
    
  
  
  
  
  class Person
  {
    public $name = '';
    public $head = true;
    public $feet = true;
    public $deflected = false;
    
    function __invoke($p){ return $this->$p; }
    function __toString(){ return 'this'; } // test for reference
    
    function __construct($name){ $this->name = $name; }
    function deflect(){ $this->deflected = true; }
    
    public function shoot()
    { // If customAttack is defined, use that as the shoot resut. Otherwise shoot feet
      if(is_callable($this->customAttack)){
        return call_user_func($this->customAttack);
      }
      
      $this->feet = false;
    }
  }
  $p = new Person('Bob');
  
  $p->customAttack = 
        function(){
          
          echo $this; // Notice: Undefined variable: this
          
          #$this = new Class() // FATAL ERROR
          
          // Trick to assign the variable '$this'
          extract(array('this' => requesting_class())); // Determine what class is responsible for making the call to Closure
          
          var_dump( $this ); // Passive reference works
          var_dump( $$this ); // Added to class: function __toString(){ return 'this'; }
          
          $name = $this('name'); // Success
          echo $name;      // Outputs: Bob
          echo '<br />';
          echo $$this->name;
          
          call_user_func_array(array($this, 'deflect'), array()); // SUCCESSFULLY CALLED
          
          #$this->head = 0; //** FATAL ERROR: Using $this when not in object context
          $$this->head = 0; // Successfully sets value
          
        };
 
  print_r($p);
  
  $p->shoot();
  
  print_r($p);
  
  die();
?>

If you want to check whether you're dealing with a closure specifically and not a string or array callback you can do this:
<?php
$isAClosure = is_callable($thing) && is_object($thing);
?>

Since it is possible to assign closures to class variables, it is a shame it is not possible to call them directly. ie. the following does not work:
<?php
class foo {
 public test;
 public function __construct(){
  $this->test = function($a) {
   print "$a\n";
  };
 }
}
$f = new foo();
$f->test();
?>
However, it is possible using the magic __call function:
<?php
class foo {
 public test;
 public function __construct(){
  $this->test = function($a) {
   print "$a\n";
  };
 }
 public function __call($method, $args){
  if ( $this->{$method} instanceof Closure ) {
   return call_user_func_array($this->{$method},$args);
  } else {
   return parent::__call($method, $args);
  }
 }
}
$f = new foo();
$f->test();
?>
it 
Hope it helps someone ;)

You cannot use closures as a class variable member in its declaration like this:
<?php
class foo
{
  public $bar = function() {
    echo "baz";
  };
}
?>
I don't know why but this will raise a
Parse error: syntax error, unexpected 'function' (T_FUNCTION)
(used PHP 5.4)

use() parameters are early binding - they use the variable's value at the point where the lambda function is declared, rather than the point where the lambda function is called (late binding).
If you want late binding put & before the variable inside use()
<?php
$fn = function () use (&$var) { echo $var; };
?>
Examples:
<?php
// problem 1: this should echo "Canada", not a php notice
$fn = function () use ($country) { echo $country . "\n"; };
$country = 'Canada';
$fn();
// problem 2: this should echo "Canada", not "UnitedStates"
$country = 'UnitedStates';
$fn = function () use ($country) { echo $country . "\n"; };
$country = 'Canada';
$fn();
// problem 3: this should echo "Canada", not "UnitedStates"
$country = (object)array('name' => 'UnitedStates');
$fn = function () use ($country) { echo $country->name . "\n"; };
$country = (object)array('name' => 'Canada');
$fn();
// problem 4: this outputs "Canada". if this outputs "Canada",
// then so should problem 2 above. otherwise this should be
// just as broken as problem 2 and be outputting "UnitedStates"
$country = (object)array('name' => 'UnitedStates');
$fn = function () use ($country) { echo $country->name . "\n"; };
$country->name = 'Canada';
$fn();
?>
see http://bugs.php.net/bug.php?id=50980
(I've just quoted from there, but if you want you can read there the whole feature request)

Some comparisons of PHP and JavaScript closures.
=== Example 1 (passing by value) ===
PHP code:
<?php
$aaa = 111;
$func = function() use($aaa){ print $aaa; };
$aaa = 222;
$func(); // Outputs "111"
?>
Similar JavaScript code:
<script type="text/javascript">
var aaa = 111;
var func = (function(aaa){ return function(){ alert(aaa); } })(aaa);
aaa = 222;
func(); // Outputs "111"
</script>
Be careful, following code is not similar to previous code:
<script type="text/javascript">
var aaa = 111;
var bbb = aaa;
var func = function(){ alert(bbb); };
aaa = 222;
func(); // Outputs "111", but only while "bbb" is not changed after function declaration
// And this technique is not working in loops:
var functions = [];
for (var i = 0; i < 2; i++)
{
  var i2 = i;
  functions.push(function(){ alert(i2); });
}
functions[0](); // Outputs "1", wrong!
functions[1](); // Outputs "1", ok
</script>
=== Example 2 (passing by reference) ===
PHP code:
<?php
$aaa = 111;
$func = function() use(&$aaa){ print $aaa; };
$aaa = 222;
$func(); // Outputs "222"
?>
Similar JavaScript code:
<script type="text/javascript">
var aaa = 111;
var func = function(){ alert(aaa); };
aaa = 222; // Outputs "222"
func();
</script>

I fell uncomfortable with php's way to pass callbacks ahead. It's just a personal opinion but i think the structure array($object, 'method') a little bit ugly. In some special cases i would like to do something more verbose, like that:
<?php
  $sortedArray = ArrayServices::sort($arrayOfPerson, $sortPerson->byName);
?>
Here, "$sortPerson->byName" is a pointer to a function that receives 2 instances of Person and return true if the name of the first is "bigger" than the second. "ArrayServices::sort" is a function that can sort any kind of array using different criterias, it uses the function passed on the second parameter to compare two items on the array
I am able do that defining a simple abstract class which gives the ability to it's children to expose their methods as closures using __get() magic funtion.
<?php
abstract class ClosureExposerObject
{
  public function __get($methodName)
  {
    if (is_callable(
        array($this, $methodName)))
    {
      return function() use ($methodName)
      {
        $args = func_get_args(); 
        return call_user_func_array(array($this, $methodName), $args);
      };
    }
    else
    {
      $className = get_class($this);
      throw new \BadMethodCallException("$method is not a callable at $className class.");
    }
  }
}
?>
Now it is possible to define a class SortPerson so that i could use it like in the first code snippet.
<?php
class SortPerson extends ClosureExposerObject
{
  public function byName(Person p1, Person p2)
  {
    return p1->getName() > p2->getName();
  }
  
  public function byAge(Person p1, Person p2)
  {
    return p1->getAge() > p2->getAge();
  }
  
  public function bySurName(Person p1, Person p2)
  (...)
}
?>
I just needed to extend ClosureExposerObject and do nothing more. 
Some observations: SortPerson is a service class but i decided not to implement it with static methods. It is a personal matter but i really dont like to implement such small classes with statics. Also i have inverted some naming conventions (class names as substantives and methods as verbs) but in some very special cases i thing that it actually helps readability.

$this is currently (PHP 5.3.2) not usable directly with closures.
One can write:
<?php
$self = $this;
function () use ($self) { ... }
?>
but then the private/protected members of $this cannot be used inside the closure. This makes closures much less useful in OO code.
Until this is fixed, one can cheat using reflection:
<?php
class FullAccessWrapper
{
  protected $_self;
  protected $_refl;
  
  public function __construct($self)
  {
    $this->_self = $self;
    $this->_refl = new ReflectionObject($self);
  }
  
  public function __call($method, $args)
  {
    $mrefl = $this->_refl->getMethod($method);
    $mrefl->setAccessible(true);
    return $mrefl->invokeArgs($this->_self, $args);
  }
  
  public function __set($name, $value)
  {
    $prefl = $this->_refl->getProperty($name);
    $prefl->setAccessible(true);
    $prefl->setValue($this->_self, $value);
  }
  
  public function __get($name)
  {
    $prefl = $this->_refl->getProperty($name);
    $prefl->setAccessible(true);
    return $prefl->getValue($this->_self);
  }
  
  public function __isset($name)
  {
    $value = $this->__get($name);
    return isset($value);
  }
}
/**
 * Usage:
 * $self = giveAccess($this);
 * function() use ($self) { $self->privateMember... }
 */
function giveAccess($obj)
{
  return new FullAccessWrapper($obj);
}
// Example:
class Foo
{
  private $x = 3;
  private function f()
  {
    return 15;
  }
  
  public function getClosureUsingPrivates()
  {
    $self = giveAccess($this);
    return function () use ($self) {
      return $self->x * $self->f();
    };
  }
}
$foo = new Foo();
$closure = $foo->getClosureUsingPrivates();
echo $closure() . "\n"; // Prints 45 as expected
?>

appears kwilson at shuttlebox dot net that you may have just made unintended side effect. Note that adding the global $variable to your test function make the closure function echo second rather than first So the anonymous function works as expected with respect to globals.
<?php
  $variable = "first";
  $closure = function() {
    global $variable;
    echo $variable . "\n";
  };
  $closure();
  function test($closure)
  {
    global $variable; //Note the scope added here 
    $variable = "second";
    $closure();
  }
  test($closure);
?>
prints:
first
second
tested with php 5.3.1

Example using uasort.
<?php
// Usual method.
function cmp($a, $b) {
  return($a > $b);
}
uasort($array, 'cmp');
// New
uasort($array, function($a, $b) {
  return($a > $b);
});
?>

Beware that since PHP 5.4 registering a Closure as an object property that has been instantiated in the same object scope will create a circular reference which prevents immediate object destruction:
<?php
class Test
{
  private $closure;
  public function __construct()
  {
    $this->closure = function () {
    };
  }
  public function __destruct()
  {
    echo "destructed\n";
  }
}
new Test;
echo "finished\n";
/*
 * Result in PHP 5.3:
 * ------------------
 * destructed
 * finished
 *
 * Result since PHP 5.4:
 * ---------------------
 * finished
 * destructed
 */
?>
To circumvent this, you can instantiate the Closure in a static method:
<?php
public function __construct()
{
  $this->closure = self::createClosure();
}
public static function createClosure()
{
  return function () {
  };
}
?>

Haven't seen it documented anywhere but PHP 5.4 now allows accessing private and protected members of an object if it's passed into a lambda function:
<?php
class Scope
{
  protected $property = 'default';
  // or even
  // private $property = 'default';
  public function run()
  {
    $self = $this;
    $func = function() use ($self) {
      $self->property = 'changed';
    };
    $func();
    var_dump($this->property);
  }
}
$scope = new Scope();
$scope->run();
?>
When running the file under PHP 5.3 you get the following error message:
 * Fatal error: Cannot access protected property Scope::$property in ./file.php on line 11
PHP 5.4, however, does not complain and outputs "changed" as expected. 
Not sure why it's like that but I suspect it has something to do with 5.4 supporting passing $this into lambda functions.
Speaking of which, something to remember is that the following won't work:
<?php
$func = function() use ($this) {
  $this->property = 'changed';
};
?>
The error you'll get is:
 * PHP Fatal error: Cannot use $this as lexical variable in ./file.php on line 9
You have to totally omit the use statement for it to work:
<?php
$func = function() {
  $this->property = 'changed';
};
?>

Base dao class illustrating the usefulness of closures.
* Handles opening and closing of connections.
* Adds slashes sql
* Type checking of sql parameters and casts as appropriate
* Provides hook for processing of result set and emitting one or more objects.
* Provides hook for accessing underlying link and result objects.
<?php
define("userName","root");
define("password","root");
define("dbName","ahcdb");
define("hostName","localhost");
class BaseDao {
  function getConnection()  {
    $link = mysql_connect(hostName, userName, password);
    if (!$link) 
      die("Could not connect: " . mysql_error());
    if (!mysql_select_db(dbName))
      die("Could not select database: " . mysql_error());
    return $link;
  }
  
  function setParams(& $sql, $params)  {
    if($params != null)
      $sql = vsprintf($sql, array_map(function($n) {
        if(is_int($n))
          return (int)$n;
        if(is_float($n))
          return (float)$n;
        if(is_string($n))
          return "'".mysql_real_escape_string($n)."'";
        return mysql_real_escape_string($n);
      }, $params));
  }
  function executeQuery($sql, $params, $callback = null)  {
    $link = $this->getConnection();
    $this->setParams($sql, $params);
    $return = null;
    if(($result = mysql_query($sql, $link)) != null)
      if($callback != null)
        $return = $callback($result, $link);
    if($link != null)
      mysql_close($link);
    if(!$result)
      die("Fatal Error: Invalid query '$sql' : " . mysql_error());
    return $return;
  }
 
  function getList($sql, $params, $callback)  {
    return $this->executeQuery($sql, $params, function($result, $link) use ($callback) {
      $idx = 0;
      $list = array();
      while ($row = mysql_fetch_assoc($result))
        if($callback != null)
          $list[$idx] = $callback($idx++, $row);
      return $list;
    });
  }
  
  function getSingle($sql, $params, $callback)  {
    return $this->executeQuery($sql, $params, function($result, $link) use ($callback) {
      if ($row = mysql_fetch_assoc($result))
        $obj = $callback($row);
      return $obj;
    });
  }
}
class Example  {
  var $id;
  var $name;
  
  function Example($id, $name){
    $this->id = $id;
    $this->name = $name;
  }
  
  function setId($id){
    $this->id = $id;
  }
}
class ExampleDao extends BaseDao  {
  
  
  function getAll(){
    return parent::getList("select * from nodes", null, function($idx, $row) {
      return new Example($row["id"], $row["name"]);
    });
  }
  
  function load($id){
    return parent::getSingle("select * from nodes where id = %1\$s", array($id), function($row) {
      return new Example($row["id"], $row["name"]);
    });
  }
  
  function update($example){
    return parent::executeQuery("update nodes set name = '' where id = -1", null, function($result, $link){
      return $result;
    });
  }
  
  function insert(& $example){
    return parent::executeQuery("insert into nodes", null, function($result, $link) use ($example){
      $id = mysql_insert_id($link);
      $example->setId($id);
      return $result;
    });
  }  
}
$exampleDao = new ExampleDao();
$list = $exampleDao->getAll());
$exampleObject = $exampleDao->load(1));
$exampleDao->update($exampleObject);
?>

be aware of Fatal error: Using $this when not in object context when using in closures
http://wiki.php.net/rfc/closures/removal-of-this

Recursive anonymous function with reference in use keyword 
$r=function()use(&$r){static $i=10;echo $i.PHP_EOL;($i--)?$r():$i;};$r();

Beware of using $this in anonymous functions assigned to a static variable.
<?php
class Foo {
  public function bar() {
    static $anonymous = null;
    if ($anonymous === null) {
      // Expression is not allowed as static initializer workaround
      $anonymous = function () {
        return $this;
      };
    }
    return $anonymous();
  }
}
$a = new Foo();
$b = new Foo();
var_dump($a->bar() === $a); // True
var_dump($b->bar() === $a); // Also true
?>
In a static anonymous function, $this will be the value of whatever object instance that method was called on first.
To get the behaviour you're probably expecting, you need to pass the $this context into the function.
<?php
class Foo {
  public function bar() {
    static $anonymous = null;
    if ($anonymous === null) {
      // Expression is not allowed as static initializer workaround
      $anonymous = function (self $thisObj) {
        return $thisObj;
      };
    }
    return $anonymous($this);
  }
}
$a = new Foo();
$b = new Foo();
var_dump($a->bar() === $a); // True
var_dump($b->bar() === $a); // False
?>

One way to call a anonymous function recursively is to use the USE keyword and pass a reference to the function itself:
<?php
$count = 1;
$add = function($count) use (&$add){
  $count += 1;
  if($count < 10) $count = $add($count); //recursive calling
  return $count;
};
echo $add($count); //Will output 10 as expected
?>