<?php

$String = "àäåæËÎÏÐðÑ×÷ØÝÞßdvdá";

$String  = preg_replace("/([^a-zà-úA-ZÀ-Ú ]|[äåæËÎÏÐðÑ×÷ØÝÞß])/u", "", $String);

echo $String;