As an alternative to Johannes's suggestion for converting strings from other character sets to a 7bit representation while not just deleting latin diacritics, you might try this:
<?php
$text = iconv($from_enc, 'US-ASCII//TRANSLIT', $text);
?>
The only disadvantage is that it does not convert "ä" to "ae", but it handles punctuation and other special characters better.
--
David