You could try Zend Encoder, but it's very expensive. However, if you think your product will sell well, it might be worth the investment.
Otherwise, you may just want to copywrite your work (and if it's written in the U.S., it's automatically protected by copywrite, if I'm not mistaken). Look at vBulletin -- it's a good commercial product, but it's not encrypted, so anyone can modify it to their heart's content once they purchase it.