This code convert string Vietnam

custom.php

<?php

function convert_string_vietnam($text)
{
   $text = html_entity_decode ($text);
  $text = preg_replace("/(ä|à|á|ạ|ả|ã|â|ầ|ấ|ậ|ẩ|ẫ|ă|ằ|ắ|ặ|ẳ|ẵ)/", 'a', $text);
  $text = str_replace("ç","c",$text);
  $text = preg_replace("/(è|é|ẹ|ẻ|ẽ|ê|ề|ế|ệ|ể|ễ)/", 'e', $text);
  $text = preg_replace("/(ì|í|î|ị|ỉ|ĩ)/", 'i', $text);
  $text = preg_replace("/(ö|ò|ó|ọ|ỏ|õ|ô|ồ|ố|ộ|ổ|ỗ|ơ|ờ|ớ|ợ|ở|ỡ)/", 'o', $text);
  $text = preg_replace("/(ü|ù|ú|ụ|ủ|ũ|ư|ừ|ứ|ự|ử|ữ)/", 'u', $text);
  $text = preg_replace("/(ỳ|ý|ỵ|ỷ|ỹ)/", 'y', $text);
  $text = preg_replace("/(đ)/", 'd', $text);
  $text = preg_replace("/(Ä|À|Á|Ạ|Ả|Ã|Â|Ầ|Ấ|Ậ|Ẩ|Ẫ|Ă|Ằ|Ắ|Ặ|Ẳ|Ẵ)/", 'a', $text);
  $text = str_replace("Ç","a",$text);
  $text = preg_replace("/(È|É|Ẹ|Ẻ|Ẽ|Ê|Ề|Ế|Ệ|Ể|Ễ)/", 'e', $text);
  $text = preg_replace("/(Ì|Í|Ị|Ỉ|Ĩ)/", 'i', $text);
  $text = preg_replace("/(Ö|Ò|Ó|Ọ|Ỏ|Õ|Ô|Ồ|Ố|Ộ|Ổ|Ỗ|Ơ|Ờ|Ớ|Ợ|Ở|Ỡ)/", 'o', $text);
  $text = preg_replace("/(Ü|Ù|Ú|Ụ|Ủ|Ũ|Ư|Ừ|Ứ|Ự|Ử|Ữ)/", 'u', $text);
  $text = preg_replace("/(Ỳ|Ý|Ỵ|Ỷ|Ỹ)/", 'y', $text);
  $text = preg_replace("/(Đ)/", 'd', $text);
  $text = str_replace(" / ","-",$text);
  $text = str_replace("/","-",$text);
  $text = str_replace(" - ","-",$text);
  $text = str_replace("_","-",$text);
  $text = str_replace(" ","-",$text);
  $text = str_replace( "ß", "ss", $text);
  $text = str_replace( "&", "", $text);
  $text = str_replace( "%", "", $text);
  $text = ereg_replace("[^A-Za-z0-9-]", "", $text);
  $text = str_replace("----","-",$text);
  $text = str_replace("---","-",$text);
  $text = str_replace("--","-",$text);
  $text = preg_replace("/( |!||#|$|%|')/", '', $text);
  $text = preg_replace("/(̀|́|̉|$|>)/", '', $text);
  $text = preg_replace ("'<[\/\!]*?[^<>]*?>'si", "", $text);
  $text = strtolower($text);
  return $text;
}