protected function checksum_code93($code)
{
$chars = array('0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '-', '.', ' ', '$', '/', '+', '%', '<', '=', '>', '?');
// translate special characters
$code = strtr($code, chr(128) . chr(131) . chr(129) . chr(130), '<=>?');
$len = strlen($code);
// calculate check digit C
$p = 1;
$check = 0;
for ($i = $len - 1; $i >= 0; --$i) {
$k = array_keys($chars, $code[$i]);
$check += $k[0] * $p;
++$p;
if ($p > 20) {
$p = 1;
}
}
$check %= 47;
$c = $chars[$check];
$code .= $c;
// calculate check digit K
$p = 1;
$check = 0;
for ($i = $len; $i >= 0; --$i) {
$k = array_keys($chars, $code[$i]);
$check += $k[0] * $p;
++$p;
if ($p > 15) {
$p = 1;
}
}
$check %= 47;
$k = $chars[$check];
$checksum = $c . $k;
// resto respecial characters
$checksum = strtr($checksum, '<=>?', chr(128) . chr(131) . chr(129) . chr(130));
return $checksum;
}