array_walk_recursive itself cannot unset values. Even though you can pass array by reference, unsetting the value in the callback will only unset the variable in that scope.

<?php
/**
 * http://uk1.php.net/array_walk_recursive implementation that is used to remove nodes from the array.
 *
 * @param array The input array.
 * @param callable $callback Function must return boolean value indicating whether to remove the node.
 * @return array
 */
function walk_recursive_remove (array $array, callable $callback) {
    foreach ($array as $k => $v) {
        if (is_array($v)) {
            $array[$k] = walk_recursive_remove($v, $callback);
        } else {
            if ($callback($v, $k)) {
                unset($array[$k]);
            }
        }
    }

    return $array;
}
?>

An up to date implementation of the above function can be looked up from https://github.com/gajus/marray/blob/master/src/marray.php#L116.