is_a

(PHP 4 >= 4.2.0, PHP 5, PHP 7, PHP 8)

is_a — 如果对象属于该类或该类是此对象的父类则返回 true

说明

is_a ( mixed $object , string $class_name , bool $allow_string = false ) : bool

如果 object 是该类或该类是此对象的父类。

参数

object

类名或者实例对象。

class_name

类名

allow_string

如果本参数设置为 false，object 就不允许传入字符串类名。 这也会在类不存在时，阻止调用自动加载器（autoloader）。

返回值

如果对象属于该类或该类是此对象的父类时返回 true，否则返回 false。

范例

Example #1 is_a() 例子

<?php
// define a class
class WidgetFactory
{
  var $oink = 'moo';
}

// create a new object
$WF = new WidgetFactory();

if (is_a($WF, 'WidgetFactory')) {
  echo "yes, \$WF is still a WidgetFactory\n";
}
?>

Example #2 在 PHP 5 中使用 instanceof 运算符

<?php
if ($WF instanceof WidgetFactory) {
    echo 'Yes, $WF is a WidgetFactory';
}
?>

参见

User Contributed Notes

dkrupyanskiy[at]gmail 10-Jan-2018 09:36
Looks like the function signature given in description is wrong. Actually it can take a string  as a first parameter in the case if $allow_string is set to true.

It took some time to find out how the last parameter should be used. Please consider the following example

<?php

class Foo{}

spl_autoload_register(
    function($classname){
        printf('autoload has been triggered for %s%s', $classname, PHP_EOL);
    }
);

var_dump(is_a('UndefinedClassName', Foo::class, true));

?>
Ronald Locke 30-Sep-2016 02:19
Please note that you have to fully qualify the class name in the second parameter.

A use statement will not resolve namespace dependencies in that is_a() function.

<?php
namespace foo\bar;

class A {};
class B extends A {};
?>

<?php
namespace har\var;

use foo\bar\A;
$foo = new foo\bar\B();

is_a($foo, 'A'); // returns false;
is_a($foo, 'foo\bar\A'); // returns true;
?>

Just adding that note here because all examples are without namespaces.
portugal {at} jawira {dot} com 12-Jun-2015 09:32
I just want to point out that you can replace "is_a()" function with the "instanceof" operator, BUT you must use a variable to pass the class name string.

This will work:
<?php
$object = new \stdClass();
$class_name = '\stdClass';

var_dump(is_a($object, $class_name));     // bool(true)
var_dump(is_a($object, '\stdClass'));     // bool(true)
var_dump($object instanceof $class_name); // bool(true)
?>

While this don't:
<?php
$object = new \stdClass();
var_dump($object instanceof '\stdClass'); // Parse error: syntax error, unexpected ''\stdClass'' (T_CONSTANT_ENCAPSED_STRING)
?>
eitan at mosenkis dot net 12-Jan-2012 01:09
As of PHP 5.3.9, is_a() seems to return false when passed a string for the first argument. Instead, use is_subclass_of() and, if necessary for your purposes, also check if the two arguments are equal, since is_subclass_of('foo', 'foo') will return false, while is_a('foo', 'foo') used to return true.
Aron Budinszky 07-Sep-2011 04:21
Be careful! Starting in PHP 5.3.7 the behavior of is_a() has changed slightly: when calling is_a() with a first argument that is not an object, __autoload() is triggered!

In practice, this means that calling is_a('23', 'User'); will trigger __autoload() on "23". Previously, the above statement simply returned 'false'.

More info can be found here:
https://bugs.php.net/bug.php?id=55475

Whether this change is considered a bug and whether it will be reverted or kept in future versions is yet to be determined, but nevertheless it is how it is, for now...
p dot scheit at zweipol dot net 16-Jan-2007 01:44
At least in PHP 5.1.6 this works as well with Interfaces.

<?php
interface test {
  public function A();
}

class TestImplementor implements test {
  public function A () {
    print "A";
  }
}

$testImpl = new TestImplementor();

var_dump(is_a($testImpl,'test'));
?>

will return true
cesoid at yahoo dot com 05-Oct-2005 07:01
is_a returns TRUE for instances of children of the class.

For example:

class Animal
{}

class Dog extends Animal
{}

$test = new Dog();

In this example is_a($test, "Animal") would evaluate to TRUE as well as is_a($test, "Dog").

This seemed intuitive to me, but did not seem to be documented.