Clean way to recursivelly merge an array (secure & fast way)

merge-array.php

<?php
/**
 * An array merging helper
 * @params array $original
 * @params array $additionnal
 * @return array
 */
function merge_array( $original, $additionnal ) 
{
    if ( empty($additionnal) ) return $original;
    foreach($additionnal as $key => $value) {
        if ( is_numeric( $key ) ) {
            $original[] = $value;
        } else {
            $original[$key] = (
                !empty($original[$key]) 
                && is_array($value) 
                && is_array($original[$key]) ?
                merge_array($original[$key], $additionnal[$key]) : $value
            );
        }
    }
    return $original;
}