There are large scale applications written on both PHP and Java, which have been successful and failed. This applies not only to Java or PHP, but plethora of other programming languages out there. So the question which is a better choice is redundant. What should you choose? Well you would be the best one to assess that.
If you have experience of working in PHP, it is more likely that you would write better code in PHP than what you would make of Java. So Java threads go out of the Window; right away. If you are going to hire people to code, then it becomes tricky. If I were you, I would choose a programming language that I am comfortable with.
Some of the things that you might want to look at are
- How well do you understand to build a secure app in PHP and Java.
- How do you plan to host your app and scale it if required.
- What are existing API support that makes it easy to develop application.
- What is the time constraint. IMO prototyping and initial development is faster in PHP, compared to Java.
- What community support do you have for whatever APIs you are going to use.
Update
There are plenty of existing discussion on SO over this here, here and here. I would not want to comment much on that because its been discussed more eloquently already. You can read about benchmarking here
IMO Java is fast but could be a memory hog to begin with, you would need to know or learn about tweaking JVM parameters to run your application optimally. This could be true for PHP as well, but I think default works fine. I haven't worked on PHP extensively, but of what I have realized is that development is faster in PHP. The reason that I find PHP faster to develop is because Java is a very verbose language. So if you are starting with Java you have to be prepared to deal with it. While performance depends on how you implement your application. We have run in to similar issues irrespective of programming language in use.
