Thanks to Sonia Heimann, who found the bug and
proposed a new algo.
Versions note:
2 new public variables:
* $this->unoptimized to set to true if you
experience problems with "laposte colissimo" barcode;
* $this->nozerofill to disable trailing and leading zeros
(then it's up to you to manage blank zones)
was unable to encode single character codes.
Thanks to Joachim Reiter for his patch !