Translating JS function to PHP

Question

I'm trying to "translate" an JS function into PHP equivalent. For the following JS function:

function dF(s,n)
{
n=parseInt(n);
var s1=unescape(s.substr(0,n)+s.substr(n+1,s.length-n-1));
var t='';
for(i=0;i<s1.length;i++)
{
t+=String.fromCharCode(s1.charCodeAt(i)-s.substr(n,1));
}
return(unescape(t));
}

i did this in PHP:

function dF( $s, $n ) {
    $n1 = intval( $n );
    $s1 = urldecode( substr( $s, 0, $n1 ) . substr( $s, $n1+1, strlen( $s)-$n1-1 ));
    $t = '';
    for( $i = 0; $i < strlen($s1); $i++ ) {
        $t .= unichr(substr($s1, $i) - substr($s, $n1, 1));
    }
    return urldecode($t);
}

/**
 * Return unicode char by its code
 *
 * @param int $u
 * @return char
 */
function unichr($u) {
    return mb_convert_encoding('&#' . intval($u) . ';', 'UTF-8', 'HTML-ENTITIES');
}

And it fails somewhere around urldecode() I persume. What am I doing wrong?

--- More info ---

Result for the JS function, for example dF('8%3Fn%3FAAl%3A%3Ej%3D%3C%3B%3A%3DA%3Ail%40llk%3Aj%3B8%38El%3B%40m','55'); would be 07f799d26b5432592ad8ddc2b306d38e

But I'm not getting this as a result. After fixes from first answer, echo dF('8%3Fn%3FAAl%3A%3Ej%3D%3C%3B%3A%3DA%3Ail%40llk%3Aj%3B8%38El%3B%40m', '55') is empty, blank, nothing printed.

Answer 1

Looks to me like this is the culprit:

$t .= unichr(ord( $s1[$i] ) - substr($s, $n1, 1));
                       // ^ This

If you do substr($str, 3) it gets EVERYTHING after the third letter, not just the third letter. I assume that's what charCodeAt(...) in your JS does, utilizes only one letter?

To get only one letter, do this:

echo $str[$i]; // echos the first character of $str if $i was 0