When doing transliteration, you have to make sure that your LC_COLLATE is properly set, otherwise the default POSIX will be used.
To transform "rené" into "rene" we could use the following code snippet:
<?php
setlocale(LC_CTYPE, 'nl_BE.utf8');
$string = 'rené';
$string = iconv('UTF-8', 'ASCII//TRANSLIT', $string);
echo $string; // outputs rene
?>