The crc32_combine() function provided by petteri at qred dot fi has a bug that causes an infinite loop, a shift operation on a 32-bit signed int might never reach zero. Replacing the function gf2_matrix_times() with the following seems to fix it:

<?php
function gf2_matrix_times($mat, $vec)
{
    $sum=0;
    $i=0;

    while ($vec) {
        if ($vec & 1) {
            $sum ^= $mat[$i];
        }
        $vec = ($vec >> 1) & 0x7FFFFFFF;
        $i++;       
    }
    return $sum;
}
?>

Otherwise, it's probably the best solution if you can't use hash_file(). Using a 1meg read buffer, the function only takes twice as long to process a 300meg files than hash_file() in my test.