Flatten a two-dimensional PHP array

flatten2d.php

<?php

function flatten2d($multi_dimensional_array) {
  $flattened = [];
  $flatten = function($value, $index) { 
    global $flattened;
    array_push($flattened, $value);
  };
  array_walk_recursive($multi_dimensional_array, $flatten);
  return $flattened;
}