Username converted to us-ascii (unix-style)
If you only want to transliterate non-ASCII European characters (like those used in Nordic, German and French names), you can use the PHP function strtr. E.g:

$name = strtr($name,
   'ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ',
   'AAAAAAACEEEEIIIIDNOOOOOOUUUUYTsaaaaaaaceeeeiiiidnoooooouuuuyty');


If you want to do romanization of a wider range of non-ASCII alphabets (cyrillic, greek, etc.) take a look at the contributed project Transliteration. 

While this is specifically to transliterate file names to US-ASCII, at its core is UTF-8 normalization (based on UtfNormal.php from MediaWiki) and transliteration (based upon Sean M. Burke's Text::Unidecode).  You should be able to use the same libraries for romanization of names and other strings.