Well, there are code obfuscators out there. And you can use the Zend encryption. There's also other third-party apps out there.
For the most part, via the web, you can't download the source. Unless someone discovers a security breach in your code that allows them in via an FTP (or how they can somehow download without having the server parse the file), then third-party apps would come in handy.
Generally: you don't need it. Your source is fairly hidden.
If you are writing an app that you will distribute (like WHMAutoPilot, vBulletin, etc.) where people will be able to see your code, a code obfuscator would work great.
You can google PHP Code Encryption on google and get some results:
SourceGuardian
PHP Code Lock
IonCube
Zend Optimizer
~Brett