function array_flatten($array, $flat = false)
{
    if (!is_array($array) || empty($array)) return '';
    if (empty($flat)) $flat = array();
       
    foreach ($array as $key => $val) {
      if (is_array($val)) $flat = array_flatten($val, $flat);
      else $flat[] = $val;
    }
       
    return $flat;
}

This will outpu a numeric result array

Flatten an array in PHP

php

PHP