函数的参数

通过参数列表可以传递信息到函数，即以逗号作为分隔符的表达式列表。参数是从左向右求值的。

PHP 支持按值传递参数（默认），通过引用传递参数以及默认参数。也支持可变长度参数列表。

Example #1 向函数传递数组

<?php
function takes_array($input)
{
    echo "$input[0] + $input[1] = ", $input[0]+$input[1];
}
?>

通过引用传递参数

默认情况下，函数参数通过值传递（因而即使在函数内部改变参数的值，它并不会改变函数外部的值）。如果希望允许函数修改它的参数值，必须通过引用传递参数。

如果想要函数的一个参数总是通过引用传递，可以在函数定义中该参数的前面加上符号&：

Example #2 用引用传递函数参数

<?php
function add_some_extra(&$string)
{
    $string .= 'and something extra.';
}
$str = 'This is a string, ';
add_some_extra($str);
echo $str;    // outputs 'This is a string, and something extra.'
?>

默认参数的值

函数可以定义 C++风格的标量参数默认值，如下所示：

Example #3 在函数中使用默认参数

<?php
function makecoffee($type = "cappuccino")
{
    return "Making a cup of $type.\n";
}
echo makecoffee();
echo makecoffee(null);
echo makecoffee("espresso");
?>

以上例程会输出：

Making a cup of cappuccino.
Making a cup of .
Making a cup of espresso.

PHP 还允许使用数组array和特殊类型NULL作为默认参数，例如：

Example #4 使用非标量类型作为默认参数

<?php
function makecoffee($types = array("cappuccino"), $coffeeMaker = NULL)
{
    $device = is_null($coffeeMaker) ? "hands" : $coffeeMaker;
    return "Making a cup of ".join(", ", $types)." with $device.\n";
}
echo makecoffee();
echo makecoffee(array("cappuccino", "lavazza"), "teapot");
?>

默认值必须是常量表达式，不能是诸如变量，类成员，或者函数调用等。

注意当使用默认参数时，任何默认参数必须放在任何非默认参数的右侧；否则，函数将不会按照预期的情况工作。考虑下面的代码片断：

Example #5 函数默认参数的不正确用法

<?php
function makeyogurt($type = "acidophilus", $flavour)
{
    return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry");   // won't work as expected
?>

以上例程会输出：

Warning: Missing argument 2 in call to makeyogurt() in 
/usr/local/etc/httpd/htdocs/phptest/functest.html on line 41
Making a bowl of raspberry .

现在，比较上面的例子和这个例子：

Example #6 函数默认参数正确的用法

<?php
function makeyogurt($flavour, $type = "acidophilus")
{
    return "Making a bowl of $type $flavour.\n";
}
echo makeyogurt("raspberry");   // works as expected
?>

以上例程会输出：

Making a bowl of acidophilus raspberry.

Note:自 PHP 5 起，传引用的参数也可以有默认值。

类型声明

Note:

在PHP 5中，类型声明也被称为类型提示。

类型声明允许函数在调用时要求参数为特定类型。如果给出的值类型不对，那么将会产生一个错误：在PHP 5中，这将是一个可恢复的致命错误，而在PHP 7中将会抛出一个TypeError异常。

为了指定一个类型声明，类型应该加到参数名前。这个声明可以通过将参数的默认值设为NULL来实现允许传递NULL。

Valid types

Warning

Aliases for the above scalar types are not supported. Instead, they are treated as class or interface names. For example, usingbooleanas a parameter or return type will require an argument or return value that is aninstanceofthe class or interfaceboolean, rather than of type bool:

<?php function test(boolean $param) {} test(true); ?>

以上例程会输出：

 Fatal error: Uncaught TypeError: Argument 1 passed to test() must be an instance of boolean, boolean given, called in - on line 1 and defined in -:1
 

范例

Example #7 Basic class type declaration

<?php
class C {}
class D extends C {}
// This doesn't extend C.
class E {}
function f(C $c) {
    echo get_class($c)."\n";
}
f(new C);
f(new D);
f(new E);
?>

以上例程会输出：

C
D
Fatal error: Uncaught TypeError: Argument 1 passed to f() must be an instance of C, instance of E given, called in - on line 14 and defined in -:8
Stack trace:
#0 -(14): f(Object(E))
#1 {main}
  thrown in - on line 8

Example #8 Basic interface type declaration

<?php
interface I { public function f(); }
class C implements I { public function f() {} }
// This doesn't implement I.
class E {}
function f(I $i) {
    echo get_class($i)."\n";
}
f(new C);
f(new E);
?>

以上例程会输出：

C
Fatal error: Uncaught TypeError: Argument 1 passed to f() must implement interface I, instance of E given, called in - on line 13 and defined in -:8
Stack trace:
#0 -(13): f(Object(E))
#1 {main}
  thrown in - on line 8

Example #9 Nullable type declaration

<?php
class C {}
function f(C $c = null) {
    var_dump($c);
}
f(new C);
f(null);
?>

以上例程会输出：

object(C)#1 (0) {
}
NULL

严格类型

默认情况下，如果能做到的话，PHP将会强迫错误类型的值转为函数期望的标量类型。例如，一个函数的一个参数期望是string，但传入的是integer，最终函数得到的将会是一个string类型的值。

可以基于每一个文件开启严格模式。在严格模式中，只有一个与类型声明完全相符的变量才会被接受，否则将会抛出一个TypeError。唯一的一个例外是可以将integer传给一个期望float的函数。

使用declare语句和strict_types声明来启用严格模式：

启用严格模式同时也会影响返回值类型声明.

Note:

严格类型适用于在启用严格模式的文件内的函数调用，而不是在那个文件内声明的函数。一个没有启用严格模式的文件内调用了一个在启用严格模式的文件中定义的函数，那么将会遵循调用者的偏好（弱类型），而这个值将会被转换。

Note:

严格类型仅用于标量类型声明，也正是因为如此，这需要PHP 7.0.0 或更新版本，因为标量类型声明也是在那个版本中添加的。

Example #10 Strict typing

<?php
declare(strict_types=1);
function sum(int $a, int $b) {
    return $a + $b;
}
var_dump(sum(1, 2));
var_dump(sum(1.5, 2.5));
?>

以上例程会输出：

int(3)
Fatal error: Uncaught TypeError: Argument 1 passed to sum() must be of the type integer, float given, called in - on line 9 and defined in -:4
Stack trace:
#0 -(9): sum(1.5, 2.5)
#1 {main}
  thrown in - on line 4

Example #11 Weak typing

<?php
function sum(int $a, int $b) {
    return $a + $b;
}
var_dump(sum(1, 2));
// These will be coerced to integers: note the output below!
var_dump(sum(1.5, 2.5));
?>

以上例程会输出：

int(3)
int(3)

Example #12 Catching TypeError

<?php
declare(strict_types=1);
function sum(int $a, int $b) {
    return $a + $b;
}
try {
    var_dump(sum(1, 2));
    var_dump(sum(1.5, 2.5));
} catch (TypeError $e) {
    echo 'Error: '.$e->getMessage();
}
?>

以上例程会输出：

int(3)
Error: Argument 1 passed to sum() must be of the type integer, float given, called in - on line 10

可变数量的参数列表

PHP 在用户自定义函数中支持可变数量的参数列表。在 PHP 5.6 及以上的版本中，由...语法实现；在 PHP 5.5 及更早版本中，使用函数func_num_args()，func_get_arg()，和func_get_args()。

...in PHP 5.6+

In PHP 5.6 and later, argument lists may include the...token to denote that the function accepts a variable number of arguments. The arguments will be passed into the given variable as an array; for example:

Example #13 Using...to access variable arguments

<?php
function sum(...$numbers) {
    $acc = 0;
    foreach ($numbers as $n) {
        $acc += $n;
    }
    return $acc;
}
echo sum(1, 2, 3, 4);
?>

以上例程会输出：

10

You can also use...when calling functions to unpack an array or Traversable variable or literal into the argument list:

Example #14 Using...to provide arguments

<?php
function add($a, $b) {
    return $a + $b;
}
echo add(...[1, 2])."\n";
$a = [1, 2];
echo add(...$a);
?>

以上例程会输出：

3
3

You may specify normal positional arguments before the...token. In this case, only the trailing arguments that don't match a positional argument will be added to the array generated by....

It is also possible to add a type hint before the...token. If this is present, then all arguments captured by...must be objects of the hinted class.

Example #15 Type hinted variable arguments

<?php
function total_intervals($unit, DateInterval ...$intervals) {
    $time = 0;
    foreach ($intervals as $interval) {
        $time += $interval->$unit;
    }
    return $time;
}
$a = new DateInterval('P1D');
$b = new DateInterval('P2D');
echo total_intervals('d', $a, $b).' days';
// This will fail, since null isn't a DateInterval object.
echo total_intervals('d', null);
?>

以上例程会输出：

3 days
Catchable fatal error: Argument 2 passed to total_intervals() must be an instance of DateInterval, null given, called in - on line 14 and defined in - on line 2

Finally, you may also pass variable arguments by reference by prefixing the...with an ampersand(&).

Older versions of PHP

No special syntax is required to note that a function is variadic; however access to the function's arguments must use func_num_args(),func_get_arg() and func_get_args().

The first example above would be implemented as follows in PHP 5.5 and earlier:

Example #16 Accessing variable arguments in PHP 5.5 and earlier

<?php
function sum() {
    $acc = 0;
    foreach (func_get_args() as $n) {
        $acc += $n;
    }
    return $acc;
}
echo sum(1, 2, 3, 4);
?>

以上例程会输出：

10

To experiment on performance of pass-by-reference and pass-by-value, I used this script. Conclusions are below. 
#!/usr/bin/php
<?php
function sum($array,$max){  //For Reference, use: "&$array"
  $sum=0;
  for ($i=0; $i<2; $i++){
    #$array[$i]++;    //Uncomment this line to modify the array within the function.
    $sum += $array[$i]; 
  }
  return ($sum);
}
$max = 1E7         //10 M data points.
$data = range(0,$max,1);
$start = microtime(true);
for ($x = 0 ; $x < 100; $x++){
  $sum = sum($data, $max);
}
$end = microtime(true);
echo "Time: ".($end - $start)." s\n";
/* Run times:
#  PASS BY  MODIFIED?  Time
-  -------  ---------  ----
1  value   no     56 us
2  reference no     58 us
3  valuue   yes     129 s
4  reference yes     66 us
Conclusions:
1. PHP is already smart about zero-copy / copy-on-write. A function call does NOT copy the data unless it needs to; the data is
  only copied on write. That's why #1 and #2 take similar times, whereas #3 takes 2 million times longer than #4.
  [You never need to use &$array to ask the compiler to do a zero-copy optimisation; it can work that out for itself.]
2. You do use &$array to tell the compiler "it is OK for the function to over-write my argument in place, I don't need the original
  any more." This can make a huge difference to performance when we have large amounts of memory to copy.
  (This is the only way it is done in C, arrays are always passed as pointers)
3. The other use of & is as a way to specify where data should be *returned*. (e.g. as used by exec() ).
  (This is a C-like way of passing pointers for outputs, whereas PHP functions normally return complex types, or multiple answers
  in an array)
4. It's unhelpful that only the function definition has &. The caller should have it, at least as syntactic sugar. Otherwise
  it leads to unreadable code: because the person reading the function call doesn't expect it to pass by reference. At the moment,
  it's necessary to write a by-reference function call with a comment, thus:
  $sum = sum($data,$max); //warning, $data passed by reference, and may be modified.
5. Sometimes, pass by reference could be at the choice of the caller, NOT the function definitition. PHP doesn't allow it, but it
  would be meaningful for the caller to decide to pass data in as a reference. i.e. "I'm done with the variable, it's OK to stomp
  on it in memory".
*/
?>

There is a possibility to use parent keyword as type hint which is not mentioned in the documentation.
Following code snippet will be executed w/o errors on PHP version 7. In this example, parent keyword is referencing on ParentClass instead of ClassTrait.
<?php
namespace TestTypeHints;
class ParentClass
{
  public function someMethod()
  {
    echo 'Some method called' . \PHP_EOL;
  }
}
trait ClassTrait
{
  private $original;
  public function __construct(parent $original)
  {
    $this->original = $original;
  }
  protected function getOriginal(): parent
  {
    return $this->original;
  }
}
class Implementation extends ParentClass
{
  use ClassTrait;
  public function callSomeMethod()
  {
    $this->getOriginal() >someMethod();
  }
}
$obj = new Implementation(new ParentClass());
$obj->callSomeMethod();
?>
Outputs:
Some method called

A function's argument that is an object, will have its properties modified by the function although you don't need to pass it by reference.
<?php
$x = new stdClass();
$x->prop = 1;
function f ( $o ) // Notice the absence of &
{
 $o->prop ++;
}
f($x);
echo $x->prop; // shows: 2
?>
This is different for arrays:
<?php
$y = [ 'prop' => 1 ];
function g( $a )
{
 $a['prop'] ++;
 echo $a['prop']; // shows: 2
}
g($y);
echo $y['prop']; // shows: 1
?>

There are fewer restrictions on using ... to supply multiple arguments to a function call than there are on using it to declare a variadic parameter in the function declaration. In particular, it can be used more than once to unpack arguments, provided that all such uses come after any positional arguments.
<?php
$array1 = [[1],[2],[3]];
$array2 = [4];
$array3 = [[5],[6],[7]];
$result = array_merge(...$array1); // Legal, of course: $result == [1,2,3];
$result = array_merge($array2, ...$array1); // $result == [4,1,2,3]
$result = array_merge(...$array1, $array2); // Fatal error: Cannot use positional argument after argument unpacking.
$result = array_merge(...$array1, ...$array3); // Legal! $result == [1,2,3,5,6,7]
?>
The Right Thing for the error case above would be for $result==[1,2,3,4], but this isn't yet (v7.1.8) supported.

In function calls, PHP clearly distinguishes between missing arguments and present but empty arguments. Thus:
<?php
function f( $x = 4 ) { echo $x . "\\n"; }
f(); // prints 4
f( null ); // prints blank line
f( $y ); // $y undefined, prints blank line
?>
The utility of the optional argument feature is thus somewhat diminished. Suppose you want to call the function f many times from function g, allowing the caller of g to specify if f should be called with a specific value or with its default value:
<?php
function f( $x = 4 ) {echo $x . "\\n"; }
// option 1: cut and paste the default value from f's interface into g's
function g( $x = 4 ) { f( $x ); f( $x ); }
// option 2: branch based on input to g
function g( $x = null ) { if ( !isset( $x ) ) { f(); f() } else { f( $x ); f( $x ); } }
?>
Both options suck.
The best approach, it seems to me, is to always use a sentinel like null as the default value of an optional argument. This way, callers like g and g's clients have many options, and furthermore, callers always know how to omit arguments so they can omit one in the middle of the parameter list.
<?php
function f( $x = null ) { if ( !isset( $x ) ) $x = 4; echo $x . "\\n"; }
function g( $x = null ) { f( $x ); f( $x ); }
f(); // prints 4
f( null ); // prints 4
f( $y ); // $y undefined, prints 4
g(); // prints 4 twice
g( null ); // prints 4 twice
g( 5 ); // prints 5 twice
?>

PASSING A "VARIABLE-LENGTH ARGUMENT LIST OF REFERENCES" TO A FUNCTION
As of PHP 5, Call-time pass-by-reference has been deprecated, this represents no problem in most cases, since instead of calling a function like this:
  myfunction($arg1, &$arg2, &$arg3);
you can call it
  myfunction($arg1, $arg2, $arg3);
provided you have defined your function as 
  function myfuncion($a1, &$a2, &$a3) { // so &$a2 and &$a3 are 
                               // declared to be refs.
  ... <function-code>
  }
However, what happens if you wanted to pass an undefined number of references, i.e., something like:
  myfunction(&$arg1, &$arg2, ..., &$arg-n);?
This doesn't work in PHP 5 anymore.
In the following code I tried to amend this by using the 
array() language-construct as the actual argument in the 
call to the function.
<?php
 function aa ($A) {
  // This function increments each 
  // "pseudo-argument" by 2s
  foreach ($A as &$x) { 
   $x += 2;
  }
 }
 
 $x = 1; $y = 2; $z = 3;
 
 aa(array(&$x, &$y, &$z));
 echo "--$x--$y--$z--\n";
 // This will output:
 // --3--4--5--
?>
I hope this is useful.
Sergio.

You can use (very) limited signatures for your functions, specifing type of arguments allowed. 
For example:
public function Right( My_Class $a, array $b )
tells first argument have to by object of My_Class, second an array. My_Class means that you can pass also object of class that either extends My_Class or implements (if My_Class is abstract class) My_Class. If you need exactly My_Class you need to either make it final, or add some code to check what $a really.
Also note, that (unfortunately) "array" is the only built-in type you can use in signature. Any other types i.e.:
public function Wrong( string $a, boolean $b )
will cause an error, because PHP will complain that $a is not an *object* of class string (and $b is not an object of class boolean).
So if you need to know if $a is a string or $b bool, you need to write some code in your function body and i.e. throw exception if you detect type mismatch (or you can try to cast if it's doable).

Editor's note: what is expected here by the parser is a non-evaluated expression. An operand and two constants requires evaluation, which is not done by the parser. However, this feature is included as of PHP 5.6.0. See this page for more information: http://php.net/migration56.new-features#migration56.new-features.const-scalar-exprs
--------
"The default value must be a constant expression" is misleading (or even wrong). PHP 5.4.4 fails to parse this function definition:
function htmlspecialchars_latin1($s, $flags = ENT_COMPAT | ENT_HTML401) {}
This yields a " PHP Parse error: syntax error, unexpected '|', expecting ')' " although ENT_COMPAT|ENT_HTML401 is certainly what a compiler-affine person would call a "constant expression".
The obvious workaround is to use a single special value ($flags = NULL) as the default, and to set it to the desired value in the function's body (if ($flags === NULL) { $flags = ENT_COMPAT | ENT_HTML401; }).

As of PHP 5.6, you can use an array as arguments when calling a function with the ... $args syntax.
<?php
  $args = array('wu','WU','wuxiancheng.cn');
  $string = str_replace(...$args);
  echo $string;
?>
Ha ha, is that interesting and powerful? 
Also you can use it like this
<?php
  $args = array('WU','wuxiancheng.cn');
  $string = str_replace('wu', ...$args);
  echo $string;
?>
It also can be used to define a user function.
<?php
  function wxc ($arg1, $arg2, ...$otherArgs){
    echo '<pre>';
    print_r($otherArgs);
    print_r(func_get_args());
    echo '</pre>';
  }
  wxc (1, 2, ...array(3,4,5));
?>
REMEMBER this: ... $args is not supported in PHP 5.5 and older versions.

You can use a class constant as a default parameter.
<?php
class A {
  const FOO = 'default';
  function bar( $val = self::FOO ) {
    echo $val;
  }
}
$a = new A();
$a->bar(); // Will echo "default"

Nothing was written here about argument types as part of the function definition.
When working with classes, the class name can be used as argument type. This acts as a reminder to the user of the class, as well as a prototype for php control. (At least in php 5 -- did not check 4).
<?php
class foo {
 public $data;
 public function __construct($dd)
 {
  $this->data = $dd;
 }
};
class test {
 public $bar;
 public function __construct(foo $arg) // Strict typing for argument
 {
  $this->bar = $arg;
 }
 public function dump()
 {
  echo $this->bar->data . "\n";
 }
};
$A = new foo(25);
$Test1 = new test($A);
$Test1->dump();
$Test2 = new test(10); // wrong argument for testing
?>
outputs:
25
PHP Fatal error: Argument 1 passed to test::__construct() must be an object of class foo, called in testArgType.php on line 27 and defined in testArgType.php on line 13

I wondered if variable length argument lists and references works together, and what the syntax might be. It is not mentioned explicitly yet in the php manual as far as I can find. But other sources mention the following syntax "&...$variable" that works in php 5.6.16. 
<?php
function foo(&...$args)
{
  $i = 0;
  foreach ($args as &$arg) {
    $arg = ++$i;
  }
}
foo($a, $b, $c);
echo 'a = ', $a, ', b = ', $b, ', c = ', $c;
?>
Gives
a = 1, b = 2, c = 3

There is a nice trick to emulate variables/function calls/etc as default values:
<?php
$myVar = "Using a variable as a default value!";
function myFunction($myArgument=null) {
  if($myArgument===null)
    $myArgument = $GLOBALS["myVar"];
  echo $myArgument;
}
// Outputs "Hello World!":
myFunction("Hello World!");
// Outputs "Using a variable as a default value!":
myFunction();
// Outputs the same again:
myFunction(null);
// Outputs "Changing the variable affects the function!":
$myVar = "Changing the variable affects the function!";
myFunction();
?>
In general, you define the default value as null (or whatever constant you like), and then check for that value at the start of the function, computing the actual default if needed, before using the argument for actual work.
Building upon this, it's also easy to provide fallback behaviors when the argument given is not valid: simply put a default that is known to be invalid in the prototype, and then check for general validity instead of a specific value: if the argument is not valid (either not given, so the default is used, or an invalid value was given), the function computes a (valid) default to use.

Quote:
"The declaration can be made to accept NULL values if the default value of the parameter is set to NULL."
But you can do this (PHP 7.1+):
<?php
function foo(?string $bar) {
  //...
}
foo(); // Fatal error
foo(null); // Okay
foo('Hello world'); // Okay
?>

PHP 7+ does type coercion if strict typing is not enabled, but there's a small gotcha: it won't convert null values into anything.
You must explicitly set your default argument value to be null (as explained in this page) so your function can also receive nulls.
For instance, if you type an argument as "string", but pass a null variable into it, you might expect to receive an empty string, but actually, PHP will yell at you a TypeError.
<?php
function null_string_wrong(string $str) {
 var_dump($str);
}
function null_string_correct(string $str = null) {
 var_dump($str);
}
$null = null;
null_string_wrong('a');   //string(1) "a"
null_string_correct('a');  //string(1) "a"
null_string_correct();   //NULL
null_string_correct($null); //NULL
null_string_wrong($null);  //TypeError thrown
?>

With regards to:
It is also possible to force a parameter type using this syntax. I couldn't see it in the documentation.
function foo(myclass par) { }
I think you are referring to Type Hinting. It is documented here: http://ch2.php.net/language.oop5.typehinting

If you use ... in a function's parameter list, you can use it only once for obvious reasons. Less obvious is that it has to be on the LAST parameter; as the manual puts it: "You may specify normal positional arguments BEFORE the ... token. (emphasis mine).
<?php
function variadic($first, ...$most, $last)
{/*etc.*/}
variadic(1, 2, 3, 4, 5);
?>
results in a fatal error, even though it looks like the Thing To Do™ would be to set $first to 1, $most to [2, 3, 4], and $last to 5.

You can pass a function as an argument too.
<?php 
  function sum($numbers){
    $acc = 0;    
    foreach ($numbers as $key => $value) {
      $acc += $value;
    }
    return $acc;
  }
  function generateString(){
    $x = array(1,2,3,4,5,6,7);
    return $x;
  }
  
  echo sum(generateString());
?>

Nullable arguments:
<?php
class C {}
function foo(?C $a)
{
  var_dump($a);
}
foo(null);
foo();
?>
Output:
NULL
PHP Warning: Uncaught ArgumentCountError: Too few arguments to function foo(), 0 passed in php shell code on line 1 and exactly 1 expected in php shell code:1
Stack trace:
#0 php shell code(1): foo()
#1 {main}
 thrown in php shell code on line 1
Usage of "?" Is also possible with "string", "int", "array" and so on primitive types (which is strange). Also unliKe "= null" "?" can be passed not only for tail of arguments, e.g.:
<?php
function foo(?string $a, string $b) {}
?>

This might be documented somewhere OR obvious to most, but when passing an argument by reference (as of PHP 5.04) you can assign a value to an argument variable in the function call. For example:
function my_function($arg1, &$arg2) {
 if ($arg1 == true) {
  $arg2 = true;
 }
}
my_function(true, $arg2 = false);
echo $arg2;
outputs 1 (true)
my_function(false, $arg2 = false);
echo $arg2;
outputs 0 (false)

by default Classes constructor does not have any arguments. Using small trick with func_get_args() and other relative functions constructor becomes a function w/ args (tested in php 5.1.2). Check it out:
class A {
  public function __construct() {
    echo func_num_args() . "<br>";
    var_dump( func_get_args());
    echo "<br>";
  }
}
$oA = new A();
$oA = new A( 1, 2, 3, "txt");
Output:
0
array(0) { }
4
array(4) { [0]=> int(1) [1]=> int(2) [2]=> int(3) [3]=> string(3) "txt" }

I like to pass an associative array as an argument. This is reminiscent of a Perl technique and can be tested with is_array. For example:
<?php
function div( $opt )
{
  $class = '';
  $text = '';
  if( is_array( $opt ) )
  {
    foreach( $opt as $k => $v )
    {
      switch( $k )
      {
        case 'class': $class = "class = '$v'";
                 break;
        case 'text': $text = $v;
                 break;
      }
    }
  }
  else
  {
    $text = $opt;
  }
  return "<div $class>$text</div>";
}
?>

If class have __toString method then his methods may set return type 'string' and return self:
<?php
class Str
{
  public function getAsString(): string
  {
    return $this;
  }
  
  public function getAsObject(): self
  {
    return $this;
  }
  
  public function __toString()
  {
    return get_class($this);
  }
}
 ?>

You can use the class/interface as a type even if the class/interface is not defined yet or the class/interface implements current class/interface.
<?php
interface RouteInterface
{
  public function getGroup(): ?RouteGroupInterface;
}
interface RouteGroupInterface extends RouteInterface
{
  public function set(RouteInterface $item);
}
?>
'self' type - alias to current class/interface, it's not changed in implementations. This code looks right but throw error:
<?php
class Route
{
  protected $name;
   // method must return Route object
  public function setName(string $name): self
  {
     $this->name = $name;
     return $this;
  }
}
class RouteGroup extends Route
{
  // method STILL must return only Route object
  public function setName(string $name): self
  {
     $name .= ' group';
     return parent::setName($name);
  }
}
?>

Notice that only order matters. Example:
function f($x='a', $y='b'){
  $output = $x.$y;
  echo $output;
}
f($y='c',$x='d');
This will print 'cd'. 
If we call function like this 
f($y='c')
actually $x in function gets value of 'c' and $y takes default value, as it just fills parameters in order specified so output will be 'cb'. Names have no meaning in function call. This may be confusing if you are coming from python for example.

Call-time pass-by-ref arguments are deprecated and may not be supported later, so doing this:
----
function foo($str) {
  $str = "bar";
}
$mystr = "hello world";
foo(&$mystr);
----
will produce a warning when using the recommended php.ini file. The way I ended up using for optional pass-by-ref args is to just pass an unused variable when you don't want to use the resulting parameter value:
----
function foo(&$str) {
  $str = "bar";
}
foo($_unused_);
----
Note that trying to pass a value of NULL will produce an error.

This may be helpful when you need to call an arbitrary function known only at runtime:
You can call a function as a variable name.
<?php
function foo(){
  echo"\nfoo()";
}
function callfunc($x, $y = '')
{
  if( $y=='' )
  {
    if( $x=='' )
       echo "\nempty";
    else $x();
  }
  else
     $y->$x();
}
class cbar {
  public function fcatch(){ echo "\nfcatch"; }
}
$x = '';
callfunc($x);
$x = 'foo';
callfunc($x);
$o = new cbar();
$x = 'fcatch';
callfunc($x, $o);
echo "\n\n";
?>
The code will output
empty
foo()
fcatch

Hey, 
I started to learn for the Zend Certificate exam a few days ago and I got stuck with one unanswered-well question. 
This is the question: 
“Absent any actual need for choosing one method over the other, does passing arrays by value to a read-only function reduce performance compared to passing them by reference?’
This question answered by Zend support team at Zend.com: 
"A copy of the original $array is created within the function scope. Once the function terminates, the scope is removed and the copy of $array with it." (By massimilianoc)
Have a nice day! 
Shaked KO

Concerning default values for arguments passed by reference:
I often use that trick:
func($ref=$defaultValue) {
  $ref = "new value";
}
func(&$var);
print($var) // echo "new value"
Setting $defaultValue to null enables you to write functions with optional arguments which, if given, are to be modified.

Follow up to resource passing:
It appears that if you have defined the resource in the same file
as the function that uses it, you can get away with the global trick.
Here's the failure case:
 include "functions_doing_globals.php"
 $conn = openDatabaseConnection();
 invoke_function_doing_global_conn();
...that it fails.
Perhaps it's some strange scoping problem with include/require, or
globals trying to resolve before the variable is defined, rather
than at function execution.

Of course you can fake a global variable for a default argument by something like this:
<?php
function self_url($text, $page, $per_page = NULL) {
 $per_page = ($per_page == NULL) ? $GLOBALS['gPER_PAGE'] : $per_page; # setup a default value of per page
 return sprintf("<a href=%s?page=%s&perpage=%s>%s</a>", $_SERVER["PHP_SELF"], $page, $per_page, $text);
}
?>