How to Flatten a Multidimensional Array?

$a = array(1,2,array(3,4, array(5,6,7), 8), 9);

$it = new RecursiveIteratorIterator(new RecursiveArrayIterator($a));

foreach($it as $v) {

  echo $v, " ";

}

1 2 3 4 5 6 7 8 9

function flatten(array $array) {

    $return = array();

    array_walk_recursive($array, function($a) use (&$return) { $return[] = $a; });

    return $return;

}