Description:
------------
It would be useful to optionally be able define a return type for a function, perhaps like so:

function foo($arg1, $arg2) returns array { ... }

Currently the Code by Contract principle is not fully implementable in interfaces because there is no way to guarantee a return type for functions, or even any return value at all. 

Adding a return type would allow one to do better code by contract:

interface iMyInterface {
  function handle_data($arg1) returns iMyResponse;
}
interface iMyResponse { ... } 

When writing code that implements or depends on iMyInterface, I can know what type I will be getting out of the handle_data method.

Optionally one could specify multiple return types:

function my_string_function($string) returns string, boolean {...}

As the author of the function, I will know if I have a logic error when this function I've written doesn't return the pre-defined types and I get an error. 

As a client programer (who will later use the above function, but hasn't written it), I will know at a glance what output I can expect from a function, without scanning through the body to deduce it. 

For backwards compatibility, flexibility and php-ishness, I don't think this should be required, but optional.

function my_string_function_1() {...}
function my_string_function_2() returns string, boolean {...}
function my_string_function_3() returns string, boolean, array {...}



Test script:
---------------
<?php

function my_string_function($string) returns array, string, boolean {
  // do nothing; syntax presently invalid.
}