PHP: interface_exists - Manual

Example #1 interface_exists() example

<?php
// Check the interface exists before trying to use it
if (interface_exists('MyInterface')) {
    class MyClass implements MyInterface
    {
        // Methods
    }
}

?>

Class and Interface share SAME namespace!

class k{}

interface k {}    // Fatal error: Cannot redeclare class k

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);
?>

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.

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.