类型运算符

instanceof用于确定一个 PHP 变量是否属于某一类class的实例：

Example #1 对类使用instanceof

<?php
class MyClass
{
}
class NotMyClass
{
}
$a = new MyClass;
var_dump($a instanceof MyClass);
var_dump($a instanceof NotMyClass);
?>

以上例程会输出：

bool(true)
bool(false)

instanceof　也可用来确定一个变量是不是继承自某一父类的子类的实例：

Example #2 对继承类使用instanceof

<?php
class ParentClass
{
}
class MyClass extends ParentClass
{
}
$a = new MyClass;
var_dump($a instanceof MyClass);
var_dump($a instanceof ParentClass);
?>

以上例程会输出：

bool(true)
bool(true)

检查一个对象是否不是某个类的实例，可以使用逻辑运算符not。

Example #3 使用instanceof检查对象不是某个类的实例

<?php
class MyClass
{
}
$a = new MyClass;
var_dump(!($a instanceof stdClass));
?>

以上例程会输出：

bool(true)

最后，instanceof也可用于确定一个变量是不是实现了某个接口的对象的实例:

Example #4 对接口使用instanceof

<?php
interface MyInterface
{
}
class MyClass implements MyInterface
{
}
$a = new MyClass;
var_dump($a instanceof MyClass);
var_dump($a instanceof MyInterface);
?>

以上例程会输出：

bool(true)
bool(true)

虽然instanceof通常直接与类名一起使用，但也可以使用对象或字符串变量：

Example #5 对其它变量使用instanceof

<?php
interface MyInterface
{
}
class MyClass implements MyInterface
{
}
$a = new MyClass;
$b = new MyClass;
$c = 'MyClass';
$d = 'NotMyClass';
var_dump($a instanceof $b); // $b is an object of class MyClass
var_dump($a instanceof $c); // $c is a string 'MyClass'
var_dump($a instanceof $d); // $d is a string 'NotMyClass'
?>

以上例程会输出：

bool(true)
bool(true)
bool(false)

如果被检测的变量不是对象，instanceof 并不发出任何错误信息而是返回FALSE。不允许用来检测常量。

Example #6 用instanceof检测其它变量

<?php
$a = 1;
$b = NULL;
$c = imagecreate(5, 5);
var_dump($a instanceof stdClass); // $a is an integer
var_dump($b instanceof stdClass); // $b is NULL
var_dump($c instanceof stdClass); // $c is a resource
var_dump(FALSE instanceof stdClass);
?>

以上例程会输出：

bool(false)
bool(false)
bool(false)
PHP Fatal error:  instanceof expects an object instance, constant given

然而 instanceof 的使用还有一些陷阱必须了解。在 PHP 5.1.0 之前，如果要检查的类名称不存在，instanceof会调用__autoload()。另外，如果该类没有被装载则会产生一个致命错误。可以通过使用动态类引用或用一个包含类名的字符串变量来避开这种问题：

Example #7 避免 PHP 5.0 中 instanceof 引起的类名查找和致命错误问题

<?php
$d = 'NotMyClass';
var_dump($a instanceof $d); // no fatal error here
?>

以上例程会输出：

bool(false)

instanceof运算符是 PHP 5 引进的。在此之前用is_a()，但是后来is_a()被废弃而用instanceof替代了。注意自 PHP 5.3.0 起，又恢复使用is_a()了。

参见get_class()和is_a()。

Posting this so the word typeof appears on this page, so that this page will show up when you google 'php typeof'. ...yeah, former Java user.

You are also able to compare 2 objects using instanceOf. In that case, instanceOf will compare the types of both objects. That is sometimes very useful:
<?php
class A { }
class B { }
$a = new A;
$b = new B;
$a2 = new A;
echo $a instanceOf $a; // true
echo $a instanceOf $b; // false
echo $a instanceOf $a2; // true
?>

I don't see any mention of "namespaces" on this page so I thought I would chime in. The instanceof operator takes FQCN as second operator when you pass it as string and not a simple class name. It will not resolve it even if you have a `use MyNamespace\Bar;` at the top level. Here is what I am trying to say:
## testinclude.php ##
<?php
namespace Bar1;
{
class Foo1{ }
}
namespace Bar2;
{
class Foo2{ }
}
?>
## test.php ##
<?php
include('testinclude.php');
use Bar1\Foo1 as Foo;
$foo1 = new Foo(); $className = 'Bar1\Foo1';
var_dump($foo1 instanceof Bar1\Foo1);
var_dump($foo1 instanceof $className);
$className = 'Foo';
var_dump($foo1 instanceof $className);
use Bar2\Foo2;
$foo2 = new Foo2(); $className = 'Bar2\Foo2';
var_dump($foo2 instanceof Bar2\Foo2);
var_dump($foo2 instanceof $className);
$className = 'Foo2';
var_dump($foo2 instanceof $className);
?>
## stdout ##
bool(true)
bool(true)
bool(false)
bool(true)
bool(true)
bool(false)

Checking an object is not an instance of a class, example #3 uses extraneous parentheses.
<?php
var_dump(!($a instanceof stdClass));
?>
Because instanceof has higher operator precedence than ! you can just do
<?php
var_dump( ! $a instanceof stdClass );
?>

If you want to use "$foo instanceof $bar" to determine if two objects are the same class, remember that "instanceof" will also evaluate to true if $foo is an instance of a _subclass_ of $bar's class.
If you really want to see if they are the _same_ class, then they both have to be instances of each other's class. That is:
<?php
($foo instanceof $bar && $bar instanceof $foo)
?>
Consider it an alternative to "get_class($bar) == get_class($foo)" that avoids the detour through to string lookups and comparisons.

You can use "self" to reference to the current class:
<?php
class myclass {
  function mymethod($otherObject) {
    if ($otherObject instanceof self) {
      $otherObject->mymethod(null);
    }
    return 'works!';
  }
}
$a = new myclass();
print $a->mymethod($a);
?>

Doing $a instanceof stdClass from inside a namespace will not work on its own. 
You will have to do:
<?php
if ($a instanceof \stdClass) 
?>

If you want to test if a classname is an instance of a class, the instanceof operator won't work.
<?php
$classname = 'MyClass';
if( $classname instanceof MyParentClass) echo 'Child of it';
else echo 'Not child of it';
?>
Will always output 
Not child of it
You must use a ReflectionClass :
<?php
$classname = 'MyClass';
$myReflection = new ReflectionClass($classname);
if( $myReflection->isSubclassOf('MyParentClass')) echo 'Child of it';
else echo 'Not child of it';
?>
Will output the good result.
If you're testing an interface, use implementsInterface() instead of isSublassOf().

Response to vinyanov at poczta dot onet dot pl:
You mentionned "the instanceof operator will not accept a string as its first operand". However, this behavior is absolutely right and therefore, you're misleading the meaning of an instance.
<?php 'ClassA' instanceof 'ClassB'; ?> means "the class named ClassA is an instance of the class named ClassB". This is a nonsense sentence because when you instanciate a class, you ALWAYS obtain an object. Consequently, you only can ask if an object is an instance of a class.
I believe asking if "a ClassA belongs to a ClassB" (or "a ClassA is a class of (type) ClassB") or even "a ClassA is (also) a ClassB" is more appropriate. But the first is not implemented and the second only works with objects, just like the instanceof operator.
Plus, I just have tested your code and it does absolutely NOT do the same as instanceof (extended to classes)! I can't advise anyone to reuse it. The use of <?php is_instance_of ($instanceOfA, 'ClassB'); ?> raises a warning "include_once(Object id #1.php) …" when using __autoload (trying to look for $instanceOfA as if it was a class name).
Finally, here is a fast (to me) sample function code to verify if an object or class:
<?php
function kind_of (&$object_or_class, $class)
{
  return is_object ($object_or_class) ? 
    $object_or_class instanceof $class
    : (is_subclass_of ($object_or_class $class)
      || strtolower ($object_or_class) == strtolower ($class));
} 
?>

Example #5 could also be extended to include...
var_dump($a instanceof MyInterface);
The new result would be
bool(true)
So - instanceof is smart enough to know that a class that implements an interface is an instance of the interface, not just the class. I didn't see that point made clearly enough in the explanation at the top.

use this to check instance of or you can add the else part inside the if making it nested if for dual varification.:-
<?php
class MyClass{}
class Test extends MyClass{
public function checkObjectArray(array $array,$classname,$strict=false){
if(!$strict){
foreach($array as $element){
 if(!($element instanceOf $classname)){//we can use typehint objects
INSIDE an array
 echo "false";
 echo "<br />";
 break;
 }
 else{
 print "<br />true (if)";
 }
}
}
else{
foreach($array as $element){
if(get_class($element)!=$classname){// or we can use this function to
check the classes inside this array
echo "false (else)";
}
}
}
echo "<br />true";
}
}
$ob=new Test;
$a=new MyClass();
$array=array($a);
$ob->checkObjectArray($array,'MyClass');
?>

SIMPLE, CLEAN, CLEAR use of the instanceof OPERATOR
First, define a couple of simple PHP Objects to work on -- I'll introduce Circle and Point. Here's the class definitions for both:
<?php
class Circle
{
 protected $radius = 1.0;
 /*
  * This function is the reason we are going to use the
  * instanceof operator below.
  */
 public function setRadius($r)
 {
  $this->radius = $r;
 }
 public function __toString()
 {
  return 'Circle [radius=' . $this->radius . ']';
 }
}
class Point
{
 protected $x = 0;
 protected $y = 0;
 /*
  * This function is the reason we are going to use the
  * instanceof operator below.
  */
 public function setLocation($x, $y)
 {
  $this->x = $x;
  $this->y = $y;
 }
 public function __toString()
 {
  return 'Point [x=' . $this->x . ', y=' . $this->y . ']';
 }
}
?>
Now instantiate a few instances of these types. Note, I will put them in an array (collection) so we can iterate through them quickly.
<?php
$myCollection = array(123, 'abc', 'Hello World!',
 new Circle(), new Circle(), new Circle(),
 new Point(), new Point(), new Point());
$i = 0;
foreach($myCollection AS $item)
{
 /*
  * The setRadius() function is written in the Circle class
  * definition above, so make sure $item is an instance of
  * type Circle BEFORE calling it AND to avoid PHP PMS!
  */
 if($item instanceof Circle)
 {
  $item->setRadius($i);
 }
 /*
  * The setLocation() function is written in the Point class
  * definition above, so make sure $item is an instance of 
  * type Point BEFORE calling it AND to stay out of the ER!
  */
 if($item instanceof Point)
 {
  $item->setLocation($i, $i);
 }
 echo '$myCollection[' . $i++ . '] = ' . $item . '<br>';
}
?>
$myCollection[0] = 123
$myCollection[1] = abc
$myCollection[2] = Hello World!
$myCollection[3] = Circle [radius=3]
$myCollection[4] = Circle [radius=4]
$myCollection[5] = Circle [radius=5]
$myCollection[6] = Point [x=6, y=6]
$myCollection[7] = Point [x=7, y=7]
$myCollection[8] = Point [x=8, y=8]

The PHP parser generates a parse error on either of the two lines that are commented out here. 
Apparently the 'instanceof' construct will take a string variable in the second spot, but it will NOT take a string... lame
class Bar {}
$b = new Bar;
$b_class = "Bar";
var_export($b instanceof Bar); // this is ok
var_export($b instanceof $b_class); // this is ok
//var_export($f instanceof "Bar"); // this is syntactically illegal
//var_export($f instanceof 'Bar'); // this is syntactically illegal

Please note: != is a separate operator with separate semantics. Thinking about language grammar it's kind of ridicilous to negate an operator. Of course, it's possible to negate the result of a function (like is_a()), since it isn't negating the function itself or its semantics.
instanceof is a binary operator, and so used in binary terms like this
terma instanceof termb
while ! (negation) is a unary operator and so may be applied to a single term like this
!term
And a term never consists of an operator, only! There is no such construct in any language (please correct me!). However, instanceof doesn't finally support nested terms in every operand position ("terma" or "termb" above) as negation does:
!!!!!!!!!!!!!!term == term
So back again, did you ever write
a !!!!!!!!!!!!= b
to test equivalence?

The first parameter must be an object instance.
example:
<?php
class A {}
class B extends A {}
var_dump(A::class instanceof B); // false
var_dump(B::class instanceof A); // false
var_dump(new B() instanceof A); // true

Using an undefined variable will result in an error.
If variable is in doubt, one must prequalify:
  if ( isset( $MyInstance ) and $MyInstance instanceof MyClass ) ...

Cross version function even if you are working in php4
(instanceof is an undefined operator for php4)
  function isMemberOf($classename) {
   $ver = floor(phpversion());
   if($ver > 4) {
     $instanceof = create_function ('$obj,$classname','return $obj instanceof $classname;');
     return $instanceof($this,$classname);
   } else {
     // Php4 uses lowercase for classname.
     return is_a($this, strtolower($classname));
   }
  } // end function isMemberOf

Negated instanceof doesn't seem to be documented. When I read instanceof I think of it as a compairson operator (which I suppose it's not).
<?php
class A {}
class X {}
//parse error from !
if (new X !instanceof A) {
  throw new Exception('X is not an A');
}
//proper way to negate instanceof ?
if (!(new X instanceof A)) {
  throw new Exception('X is not an A');
}
?>

You can use instanceof also for interfaces.

Sometimes you want to typehint objects INSIDE an array, but I think you can't.
Instead, you can use this function to check the classes inside this array:
<?php
public checkObjectsArray(array $array, $classname, $strict = false) {
  if (!$strict) {
    foreach ($array as $element) {
      if (!($element instanceof $classname)) {
        return false;
      }
    }
  }
  else {
    foreach ($array as $element) {
      if (get_class($element) != $classname) {
        return false;
      }
    }
  }
  return true;
}
?>

It seems like instanceOf is using a string comparison. Longer class names take longer to check in conditional statements
eg.
if ($f instanceOf HelloWorldTestClass) 
is much slower than
if ($f instanceOf HWT)

use this for cross-version development...
<?php
function is_instance_of($IIO_INSTANCE, $IIO_CLASS){
  if(floor(phpversion()) > 4){
    if($IIO_INSTANCE instanceof $IIO_CLASS){
      return true;
      }
    else{
      return false;
      }
    }
  elseif(floor(phpversion()) > 3){
    return is_a($IIO_INSTANCE, $IIO_CLASS);
    }
  else{
    return false;
    }
  }
?>