<?php
$umlaute = array("/ä/","/ö/","/ü/","/Ä/","/Ö/","/Ü/","/ß/");
$htmltag = array("&auml;","&ouml;","&uuml;","&Auml;","&Ouml;","&Uuml;","&szlig;");
// $text = Text in dem Zeichen ersetzt werden sollen
$text = preg_replace($umlaute, $htmltag, $text);
?>