In his laste blog post, Steve Hannah shares some of his "adventures with PHP compilers" that he's had in the process of finding something to help him protect his work.
I am in a situation where I need to protect some intellectual property in a PHP script. The recommended solution is to use a code obfuscator (or compiler) to encode the script. Currently the landscape leaves much to be desired in this area as far as PHP is concerned.
He talks about Zend Guard, Source Guardian and bcompiler. It's this last one he focuses on the most, noting that it's a dynamic PECL extension to be included into PHP and used, but it doesn't quite seem to work so easily. He finally manages to get something compiled, only to find it doesn't work when executed anymore.
Conclusion: bcompiler is not ready for primetime yet. Please someone correct me if I'm wrong on this point.
He had higher hopes for SourceGuardian, though, and thankfully, it seemed to pull through. His second post on the topic notes that it "worked flawlessly" when he encoded the file and just as easily when decoding and running the script as well. He highly recommends it to anyone looking to protect their PHP source code.