Description:
------------
Hi,

The aim of interfaces is to provide the same external behavior, no matter what class implements it.

Maybe it would be nice for interfaces to be able to force a default value for argument on implemented methods.
Moreover, that way we could change all method's default argument by modifying it on one place only: the interface.

Reproduce code:
---------------
interface Fetchable {
    const FETCH_ASSOC = 1;

    function fetch($fetchStyle=self::FETCH_ASSOC);
    function fetchAll($fetchStyle=self::FETCH_ASSOC);
}

class PDOLayerStatement implements Fetchable {
    /* This one works fine, but I have to know the default value on interface,
     * and even reference the interface,
     * which will raise the amount of maintenance work
     */
    public function fetch($fetchStyle=Fetchable::FETCH_ASSOC);

    /* These two won't work, sadly :( */
    public function fetch($fetchStyle) {}
    public function fetchAll($fetchStyle) {}
}

Expected result:
----------------
It would be nice if PHP could automatically give Fetchable::FETCH_ASSOC as default value for $fetchStyle in every implementing class.

Actual result:
--------------
Fatal error: Declaration of PDOLayerStatement::fetch() must be compatible with that of Fetchable::fetch() in C:\xampp\htdocs\aspyct\src\test.php