class_exists()

(PHP 4, PHP 5, PHP 7)

检查类是否已定义

说明

检查指定的类是否已定义。

参数

类名。名字的匹配是不分区大小写的。

是否默认调用__autoload。

返回值

如果由$class_name所指的类已经定义，此函数返回TRUE，否则返回FALSE。

更新日志

范例

Example #1class_exists()例子

Example #2$autoloadparameter 例子

参见

• function_exists() 如果给定的函数已经被定义就返回 TRUE
• interface_exists() 检查接口是否已被定义
• get_declared_classes() 返回由已定义类的名字所组成的数组

If you are using aliasing to import namespaced classes, take care that class_exists will not work using the short, aliased class name - apparently whenever a class name is used as string, only the full-namespace version can be used
use a\namespaced\classname as coolclass;
class_exists( 'coolclass' ) => false

If you recursively load several classes inside an autoload function (or mix manual loading and autoloading), be aware that class_exists() (as well as get_declared_classes()) does not know about classes previously loaded during the *current* autoload invocation.
Apparently, the internal list of declared classes is only updated after the autoload function is completed.

Beware: class_exists is case-INsensitive, as is class instantiation.
php > var_dump(class_exists("DomNode"));
bool(true)
php > var_dump(class_exists("DOMNode"));
bool(true)
php > var_dump(class_exists("DOMNodE"));
bool(true)
php > $x = new DOMNOdE();
php > var_dump(get_class($x));
string(7) "DOMNode"
(tested with PHP 5.5.10 on Linux)
This can cause some headaches in correlating class names to file names, especially on a case-sensitive file system.

Hi guys!
Be careful and don't forget about second boolean argument $autoload (TRUE by default) when check exists class after spl_autoload_register. Propose short example
file second.php

file index.php

Because __autoload executing much earlier than boolean returned, imho..

I'm running PHP 5.3.4 on Windows 7 and had some difficulty autoloading classes using class_exists(). In my case, when I checked for the class and it didn't exist, class_exists automatically threw a system Exception. I was also throwing my own exception resulting in an uncaught exception.

By registering an additional autoload handler function that did nothing, I was able to stop throwing the extra Exception and only throw my own.

Found this buried in some search results. I don't remember the page URL but if it would have been here it might have saved me some time!

[ >= PHP 5.3]
If you are checking if a class exists that is in a specific namespace then you have to pass in the full path to the class:
echo (class_exists("com::richardsumilang::common::MyClass")) ? "Yes" : "No";

If you have a directory of classes you want to create. (Modules in my instance)... you can do it like that

//---
Here the rule is that all modules are on the form
ModModulename.php and that the class has the same name as the file.
The $modules array has all the classes initialized after this code

If spl_autoload_register() had been called, then function will try autoload class if it does not exists.
Use instead 

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