Calls the mul function from the available math library for OpenID.
function _openid_math_powmod($x, $y, $z) {
$library = _openid_get_math_library();
switch ($library) {
case 'gmp':
return gmp_powm($x, $y, $z);
case 'bcmath':
return bcpowmod($x, $y, $z);
}
}