求个gtk的计算php版

function charCodeAt($str,$index){
$char=mb_substr($str,$index,1,'UTF-8');
if(mb_check_encoding($char,'UTF-8')){
$ret=mb_convert_encoding($char,'UTF-32BE','UTF-8');
return hexdec(bin2hex($ret));
}else{
return null;
}
}
function getGTK($skey){
$hash=5381;
$len=strlen($skey);
for($i=0;$i<$len;++$i){
$hash+=($hash<<5)+charCodeAt($skey,$i);
}
return $hash&0x7fffffff;
}