Replace Character Codes using PHP.

How to replace character codes for german using str_replace php function. Characters such as :

ä
Ä'
é
ö
Ö
ü
Ü
Other chars.

see simple example.

 <?php
   $find ="ü";
   $rep = "u";
   $str = "aüstin";
   echo str_replace($find,$rep,utf8_encode($str));
?>