用php仿照js写的一个加密方法,出现问题

原本的js是一段加密的方法

function checkform()
{
var dcode;
var dcode1;
var dcode2;
dcode=document.form1.pwd.value;
dcode1="";
dcode2=1275903582
dcode2=""+dcode2*137;
var tmpstr;
var dcodelen;
dcodelen=dcode.length;
for (i=1;i<=dcodelen;i++){
tmpstr = dcode.substring(i-1,i);
dcode1=dcode1 + String.fromCharCode(tmpstr.charCodeAt(0)-i-dcode2.substring(i-1,i));
}
document.form1.pwd.value=dcode1;
}

这是我仿照写的php代码 但始终有错

//加密
function encryption($dcode){
    $dcode1=''; 
    $dcode2=1275903582;
    $dcode2 = "".$dcode2 * 137; 
    $tmpstr='';  
    for ($i=1;$i<=count($dcode);$i++)
    { 
            $tmpstr = substr($dcode,$i-1,1); 
            $tmpstr = $this->charCodeAt($tmpstr, 0); 
            $num = $this->fromCharCode($tmpstr-$i-substr($dcode2,$i-1,1)); 
            $dcode1=$dcode1 + $num;
    }  
            $encryDcode=$dcode1; 
            print_r($encryDcode); 
            die(); 
            return $encryDcode; 
    } 
function fromCharCode($codes) {
if (is_scalar($codes)) 
$codes= func_get_args();
$str= '';
foreach ($codes as $code)
$str.= chr($code);
return $str;
}
function charCodeAt($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; 
}
}