Hi what you need is something liek ioncube encoder. They allow you to encode your script so that only the ioncube loaders will run it. code is encrypted to prevent stealing.

Ioncube offer an encryption service based on number of lines of code so keeps cost low.

http://www.ioncube.com/online_encoder.php

Hi,
actually you can compile the PHP code using Phalanger - the PHP language compiler. You can find it on phalanger.codeplex.com/ or php-compiler.net . This compiler+runtime compiles the code into .NET assembly, checks the syntactic and some semantic errors and optimizes the code.