Custom filter :: converting umlaute to entities

hi there!

i want to convert all characters like ö, ä, ü ("umlaute") to their entities (to be used in my flash frontend).
for that i thought about using the modul "Custom filter".
can anybody tell me how to build a filter which does this conversion?

ä -> ä

or is there a shorter way by using kind of small php-codesnippet?

thanx!