Description:
------------
The check for whether a class is valid or not is dependent on what order the interfaces it implements are declared.

The example below is simple, to show the bug clearly.

A more realistic case would involve having multiple classes that implement various interfaces, including the "\Serializable" interface, and then realising that one of the user-defined interfaces should actually extend "\Serializable".

This would make some code break, depending on what order the implements interfaces was written as.

Test script:
---------------
interface Base
{
    const FOO = 5;
    function base();
}

interface Derived extends Base {
    function derived();
}

class ThisIsValid implements Base, Derived {
    public function base() {}
    public function derived() {}
}

class ThisWillError implements Derived, Base {
    public function base() {}
    public function derived() {}
}

Expected result:
----------------
No error, the definition of implements interfaces should not affect compilation.

Actual result:
--------------
Fatal error: Class ThisWillError cannot implement previously implemented interface Base in /in/Anc6g on line 18