PHP makes no difference between any class or interface or abstract class. The autoloader function you define always gets the name of the thing to autoload, and no kind of hint which one it was.
So your naming strategy cannot be autoloaded because you prefix interfaces with "interface_" and classes with "class_". Personally I find such a naming convention rather annoying.
On the other hand, your autoloader is completely unperformant. It scans whole directory trees recursively just to find one class! And the next class has to do all the work again, without the benefit of having done it before!
Please do implement a PSR-0 autoloader if you really want to do it on your own (and not use things like composer to do it for you) and stick to this naming scheme for classes and interfaces.
And please select a distinguishing classname prefix or namespace, and as a first step check inside your autoloader if the class to be loaded has this prefix. Return instantly if it has not. This frees you from having to spin the harddrive and see if the filename for the class exists.
If the prefix does not match, it is not "your" class that wants to be loaded, so your autoloader cannot know how to do it and shouldn't even try, but a different autoloader that was registered will know.