<?php

    function test (array $args) {
        $defaults = array('a' => 0, 'b' => 0, 'c' => 0);
        $args = array_merge($defaults, array_intersect_key($args, $defaults));

        list($a, $b, $c) = array_values($args);
        echo "a = $a, b = $b, c = $c\n";
    }

test(array('c' => 3));
test(array('a' => 2, 'b' => 1));