ai is quite dumb enougth, to carry even a single overflow bit after left shift

function shift_left($a, $b)
{
    $a = (int) $a;
    $b = (int) $b;

    if ($b < 0) {
        return shift_right($a, -$b);
    }

    if ($a < 0) {
        $a = (int) bcadd($a, '4294967296');
    }

    if ($b >= 31) {
        return 0;
    }

    return ($a << $b) & 0xFFFFFFFF;
}