As of 200807290201, there is no code completion support for interfaces.
I think this should be implemented:

1) Offer a list of defined interfaces in CC after writing

    class Foo implements |

2) Offer a list of defined interfaces in CC in function declaration (see Issue 142018)

3) Add code completion functionality for -> where relevant. For example:

    interface Showable {
        public function show();
    }

    function bar(Showable $sh) {
        $sh->|
    }

Code completion should offer show() method here, since interface in type hinting in function declaration is valid, and 
used.