interface_exists()

(PHP 5 >= 5.0.2, PHP 7)

检查接口是否已被定义

说明

interface_exists(string $interface_name[,bool $autoload= true]): bool

检查接口是否已被定义。

参数

$interface_name: 接口名。
$autoload: 默认是否调用__autoload。

返回值

本函数在由$interface_name给出的接口已定义时返回TRUE，否则返回FALSE。

范例

Example #1interface_exists()例子

<?php
// 在尝试使用前先检查接口是否存在
if (interface_exists('MyInterface')) {
    class MyClass implements MyInterface
    {
        // Methods
    }
}
?>

参见

get_declared_interfaces() 返回一个数组包含所有已声明的接口
class_implements() 返回指定的类实现的所有接口。
class_exists() 检查类是否已定义

As far as I remember interface_exists() was added in 5.0.2 . In 5.0.0 and 5.0.1 class_exists() used to return TRUE when asked for a existing interface. Starting 5.0.2 class_exists() doesn't do that anymore.

Class and Interface share SAME namespace!
class k{}
interface k {}  // Fatal error: Cannot redeclare class k

If you want to check for included interface and you already register spl autoloader - it will crash. Becassue autoloader trying to load `string` and he doesnt matter is it class or not.
Iv found several ways :
1 - unregister AL - -> check for Ifaces - -> register Autoloader
2 - $ifaces = array_flip(get_declared_interfaces()); 
 if($ifaces["MyIface"]) // empty // isset .
Interfaces are not bad, you can build correct geomentry of system , with validation by funcs / vars / const .
Also they are good to storage variables <?php
 ROOT::THEMES ; ROOT::LOC ; ?> . Much faster then Define, but you cant put algorithms inside, only complite strings / __file__ / etc.

A little note on namespaces that may be obvious to some, but was not obvious to me.
Although you can make the below statement when the statement is in the same namespace as the interface/class declaration MyInterface... 
<?php
$foo instanceof MyInterface
?>
Making use of the interface_exists, or class_exists functions, you must enter the full namespaced interface name like so (even if the function call is from the same namespace.)
<?php
interface_exists(__NAMESPACE__ . '\MyInterface', false);
?>