$voyelles = array("é", "è");
$phrase = str_replace($voyelles, "e", $phrase);

$voyelles = array("à", "â", "ä");
$phrase = str_replace($voyelles, "a", $phrase);

$voyelles = array("ç");
$phrase = str_replace($voyelles, "c", $phrase);

$voyelles = array(" ", ",", ";", "'", "?", "!", ".");
$phrase = str_replace($voyelles, "_", $phrase);

echo $phrase;

Il n'y a pas le plus simple?

$accents = "ÀÁÂÃÄÅàáâãäåÒÓÔÕÖØòóôõöøÈÉÊËèéêëÇçÌÍÎÏìíîïÙÚÛÜùúûüÿÑñ";
$ssaccents = "AAAAAAaaaaaaOOOOOOooooooEEEEeeeeCcIIIIiiiiUUUUuuuuyNn";
$chaine = strtr($chaine,$accents,$ssaccents);

$in = array(' ', '?', '!', '.', ',', ':', "'", '&', '(', ')', '"', '-');
$out = array('_', '', '', '', '', '', '', '_', '', '','_');
$chaine = str_replace($in, $out, $chaine);