php教程

is_a

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

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 ( object $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

The tested object

class_name

The class name

allow_string

If this parameter set to FALSE, string class name as object is not allowed. This also prevents from calling autoloader if the class doesn't exist.

返回值

Returns TRUE if the object is of this class or has this class as one of its parents, FALSE otherwise.

更新日志

版本 说明
5.3.9 Added allow_string parameter
5.3.0 This function is no longer deprecated, and will therefore no longer throw E_STRICT warnings.
5.0.0 This function became deprecated in favour of the instanceof operator. Calling this function will result in an E_STRICT warning.

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.

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