array_flatten_with_lambda_and_closure.php

$z = array(
  array(
    1,array(9,9,9),2,3,4
  ),
  array(
    3,5,6
  )
);
$array_flatten = function($array, $flatArray = array()) {
  array_walk_recursive(
    $array,
    function($value) use (&$flatArray) {
      array_push($flatArray, $value);
    }
  );
  return $flatArray;
};
var_dump(call_user_func($array_flatten,$z));