Earlier I mentioned how to fix a problem if you haven't got bcmod installed on your server and you are too idle to install it. I think it may have been a bit of a bodge.
Below is a better fix from php.net. Just define the bcmod function on the page above where you need to use it.
function bcmod( $x, $y )
$take = 5;
$mod = '';
$a = (int)$mod.substr( $x, 0, $take );
$x = substr( $x, $take );
$mod = $a % $y;
while ( strlen($x) );