I guess this all depends on what your fatal error is.
gmp_clrbit simply sets a bit to 0.
cmp function would be similar, except you would give it two gmp_init variables to compare against. It will always return 0, < 0, or > 0 depending on if the "left" (first) is larger or smaller than the "right" (second).
$num = gmp_init("0x485");
print gmp_strval($num); // 0x481
If I had to guess though, your error is probably undefined function for gmp_init (which means you now have to install the gmp extension: http://php.ca/manual/en/gmp.setup.php)