function foo($a1 = null, $a2 = null, $a3 = null, $a4 = null){
    if ($a1 !== null) doSomethingWith($a1, 1);
    if ($a2 !== null) doSomethingWith($a2, 2);
    if ($a3 !== null) doSomethingWith($a3, 3);
    if ($a4 !== null) doSomethingWith($a4, 4);
}

function foo(params $args){
    for ($i = 0; $i < count($args); $i++)
        doSomethingWith($args[$i], $i + 1);
}