Android : Restful web application, Java or PHP?

I'm creating an Android application which will access data using a restful web application. I have quite a lot of experience with Java, but minimal experience with PHP.

Having looked online, it is difficult to determine which language is more suitable, scalable, portable etc.

I'm hoping that the Application may one day have many concurrent users and therefore I need the most suitable option.

If anyone has any experience writing a web application in either language, I'd be really interested to hear about your experiences, and any problems you faced.. i.e. for a java web application do you need a tomcat server or another embeddded to server for it to be able to run?

Thanks, for any answers, Matt.