you can also use the free php encryptor found @
http://www.byterun.com/free-php-encoder.php
true it's not silver bullet, but good enough with encryption, and i doubt anyone will try hard enough to dycrypt it.
The advantage over zend is it doesn't requires you to install anything on server, it will run on any server as is

it's
free to use, just fill your email & it will send you with code to use service. there is also software available on site with trial if you want offline version or you can even buy it if you love it so much, though i think the online service is good enough in most cases (and above all it's FREE).