php---verify if a given string is encoded in valid utf-8

function phpdigVerifyUTF8($str) {
// verify if a given string is encoded in valid utf-8
if ($str === mb_convert_encoding(mb_convert_encoding($str, "UTF-32", "UTF-8"), "UTF-8", "UTF-32"))
{
return $str;
} else {
return false;
}
}