arrayMergeDefault.php

/**
 *     $defaults = array(
 *         'a' => 2
 *         'b' => 2
 *     );
 *     $untrusted = array(
 *         'a' => 4,
 *         'c' => 4
 *     );
 *     print_r(arrayMergeDefault($default, $unstrusted));
 *
 *     // Result
 *     array('a' => 4, 'b' => 2);
 */
function arrayMergeDefault($default, $data) {
    //Get data for which a default exists
    $intersect = array_intersect_key($data, $default);
    //Get defaults which are not present in data
    $diff = array_diff_key($default, $data);
    //Arrays have different keys, return the union of the two
    return $diff + $intersect;
}