php two-dimensional array to one-dimensional array

Convert 2D array to 1D array

    protected function change($arr){
        $result = [];
        array_walk_recursive($arr, function($value) use (&$result) {
            array_push($result, $value);
        });
        return $result;
    }
  

Convert two-digit array to one-dimensional array (retain key value)

protected function changeKey($arr){

    return array_merge($arr);
}