(PHP 5 >= 5.0.2, PHP 7)
interface_exists — Checks if the interface has been defined
$interface_name
   [, bool $autoload = true
  ] )Checks if the given interface has been defined.
   Returns TRUE if the interface given by 
   interface_name has been defined, FALSE otherwise.
  
Example #1 interface_exists() example
<?php
// Check the interface exists before trying to use it
if (interface_exists('MyInterface')) {
    class MyClass implements MyInterface
    {
        // Methods
    }
}
?>