PHP string conversion skills

//G_tk计算
function getGTK($skey){
  $hash = 5381;
  for($i=0;$i<strlen($skey);++$i){
   $hash += ($hash << 5) + utf8_unicode($skey[$i]);
  }
  return $hash & 0x7fffffff;
}
function utf8_unicode($c) {
  switch(strlen($c)) {
    case 1:
    return ord($c);
    case 2:
    $n = (ord($c[0]) & 0x3f) << 6;
    $n += ord($c[1]) & 0x3f;
    return $n;
    case 3:
    $n = (ord($c[0]) & 0x1f) << 12;
    $n += (ord($c[1]) & 0x3f) << 6;
    $n += ord($c[2]) & 0x3f;
    return $n;
    case 4:
    $n = (ord($c[0]) & 0x0f) << 18;
    $n += (ord($c[1]) & 0x3f) << 12;
    $n += (ord($c[2]) & 0x3f) << 6;
    $n += ord($c[3]) & 0x3f;
    return $n;
  }
}