Take a multi-dimensional array and flatten it into a one-dimensional array.

array_flatten.php

function flatten_array($array, $return = array(), $main_key = '') {
    if ($main_key)
	$main_key = $main_key.'_';
    
    foreach ($array as $k => $item) {
        if (is_array($item))
            $return = self::flatten_array($item, $return, $main_key.$k);
	else {
	    $new_key = $main_key.$k;
		$return[$main_key.$k] = $item;
	}
    }
    return $return;
}