Try building an application in PHP which takes a data set, processes it for and hour and then stores the results for the user when they return.
Please tell me you haven't exposed said application to the Internet. This kind of processing is</is> outside the realm of PHP but if you're placing that kind of application online, you're heading for trouble.
This is where PHP and Java can interact (hence the specification request). You have your lightweight but robust web interface in PHP. When the user returns to ask their data, PHP spots the cookie and makes a call to the protected Java App Server to deliver the data, PHP acting as a go between.
"php is not scalable precisely because it is not an end to end solution."
By end to end solution one assumes you mean you can write all tiers of your application in Java (and the messaging as well). Simply using all Java doesn't make your application scalable. Java is a generalist, PHP is a specialist.
The presentation tier in particular, in web applications, tends to be where the most "lines of code" are produced (including XHTML or whatever). Do you invest a huge development effort by using a verbose, statically typed language save that time with a dynamic, minimalist langauge, designed specifically for the job? Sun have realised this. So have Oracle. Even Borland.