[PHP] How to know if string is encoded as UTF8 (Unicode?)

The method below was presented in php.net site and it works as intended for the most of time.

function isUTF8($string) {
    return (utf8_encode ( utf8_decode ( $string ) ) == $string);
}

Here is code to convert the string if string is encoded as UTF8.

function convIfUTF8($string) {
    if (utf8_encode ( utf8_decode ( $string ) ) == $string) {
        return utf8_decode ( $string );
    } else {
        return $string;
    }
}