PHP Operators instanceof (type operator)


For checking whether some object is of a certain class, the (binary) instanceof operator can be used since PHP version 5.

The first (left) parameter is the object to test. If this variable is not an object, instanceof always returns false. If a constant expression is used, an error is thrown.

The second (right) parameter is the class to compare with. The class can be provided as the class name itself, a string variable containing the class name (not a string constant!) or an object of that class.

class MyClass {

$o1 = new MyClass();
$o2 = new MyClass();
$name = 'MyClass';

// in the cases below, $a gets boolean value true
$a = $o1 instanceof MyClass;
$a = $o1 instanceof $name;
$a = $o1 instanceof $o2;

// counter examples:
$b = 'b';
$a = $o1 instanceof 'MyClass'; // parse error: constant not allowed
$a = false instanceof MyClass; // fatal error: constant not allowed
$a = $b instanceof MyClass;    // false ($b is not an object)

instanceof can also be used to check whether an object is of some class which extends another class or implements some interface:

interface MyInterface {

class MySuperClass implements MyInterface {

class MySubClass extends MySuperClass {

$o = new MySubClass();

// in the cases below, $a gets boolean value true    
$a = $o instanceof MySubClass;
$a = $o instanceof MySuperClass;
$a = $o instanceof MyInterface;

To check whether an object is not of some class, the not operator (!) can be used:

class MyClass {

class OtherClass {

$o = new MyClass();
$a = !$o instanceof OtherClass; // true

Note that parentheses around $o instanceof MyClass are not needed because instanceof has higher precedence than !, although it may make the code better readable with parentheses.


If a class does not exist, the registered autoload functions are called to try to define the class (this is a topic outside the scope of this part of the Documentation!). In PHP versions before 5.1.0, the instanceof operator would also trigger these calls, thus actually defining the class (and if the class could not be defined, a fatal error would occur). To avoid this, use a string:

// only PHP versions before 5.1.0!
class MyClass {

$o = new MyClass();
$a = $o instanceof OtherClass; // OtherClass is not defined!
// if OtherClass can be defined in a registered autoloader, it is actually
// loaded and $a gets boolean value false ($o is not a OtherClass)
// if OtherClass can not be defined in a registered autoloader, a fatal
// error occurs.

$name = 'YetAnotherClass';
$a = $o instanceof $name; // YetAnotherClass is not defined!
// $a simply gets boolean value false, YetAnotherClass remains undefined.

As of PHP version 5.1.0, the registered autoloaders are not called anymore in these situations.

Older versions of PHP (before 5.0)

In older versions of PHP (before 5.0), the is_a function can be used to determine wether an object is of some class. This function was deprecated in PHP version 5 and undeprecated in PHP version 5.3.0.