You can try PHP protect which is a free PHP obfuscator to obfuscate your PHP code. EDIT: This service is not live anymore.
It is very nice, easy to use and also free.
As for what others have written here about not using obfuscation because it can be broken etc:
I have only one thing to answer them - don't lock your house door because anyone can pick your lock.
This is exactly the case, obfuscation is not meant to prevent 100% code theft. It only needs to make it a time-consuming task so it will be cheaper to pay the original coder. Hope this helps.