What is needed for a user to download and run a PHP application?

[lithiumx]

I'd like to develop a PHP application that users would download and then could run. The application will have a web service. I assume they will need Apache, but my main question is what is needed for PHP to run on their machine? Is there something needed like the JVM in Java or the .Net framework in .Net? What is it called and how difficult is it for them to download (size, etc.).Is anything else required that I did not mention?Thank you,