...
        substr($sWk, 3, 1) &= pack('U', $u31);
        ...
        substr($lWk, 0, 1) &= pack('U', $u01);
        ...

##</code><code>##

        ...
        substr($sWk, 3, 1) &= pack('c', $u31);
        ...
        substr($lWk, 0, 1) &= pack('c', $u01);
        ...