In der Dokumentation steht:
Similar functionality can be achieved with mb_convert_encoding(), which supports ISO-8859-1 and many other character encodings.
Hier ist ein Beispiel wie du mb_convert_encoding nutzt.
// zu UTF-8
$str = mb_convert_encoding($str, "UTF-8");
// von ISO-8859-1 zu UTF-8
$str = mb_convert_encoding($str, "ISO-8859-1", "UTF-8");