It will most likely work if you substitute the raw hexcode characters with the literal values you require, like in this example:
$search = array (chr(132),chr(142),chr(148),chr(153),chr(129),chr(154),chr(225));
$replace = array ('ä','Ä','ö','Ö','ü','Ü','ß');
$newtext = str_replace($search, $replace, $text);