code: private function makeKey($block, $valContext)
      {
          $pwarray = str_repeat("\0", 64);
  
          for ($i = 0; $i < 5; ++$i) {
              $pwarray[$i] = $valContext[$i];
          }
  
          $pwarray[5] = chr($block & 0xff);
          $pwarray[6] = chr(($block >> 8) & 0xff);
          $pwarray[7] = chr(($block >> 16) & 0xff);
          $pwarray[8] = chr(($block >> 24) & 0xff);
  
          $pwarray[9] = "\x80";
          $pwarray[56] = "\x48";
  
          $md5 = new Xls\MD5();
          $md5->add($pwarray);
  
          $s = $md5->getContext();
  
          return new Xls\RC4($s);
      }
docs: Make an RC4 decryptor for the given block.
  
  @param int $block Block for which to create decrypto
  @param string $valContext MD5 context state
  
  @return Xls\RC4