PHP Performance/Scalability

Hello I have a very simple PHP application which I develop on Apache-Web-Server. However the application does not scale beyond 1500 concurrent users (150 requests/sec), even though there is plenty of hardware available (CPU is getting utilized 15%, Memory is getting utilized 15%). On a similar note, when I develop the same application in Java, it scales to more than 6000 concurrent users (600 requests/sec) on the same hardware. Any pointers on how to address this scalability issue? Thanks! Deepak "The greatness of a nation can be judged by the way its animals are treated - Mahatma Gandhi" +91 73500 12833 deicool@gmail.com Facebook: https://www.facebook.com/deicool LinkedIn: www.linkedin.com/in/deicool "Plant a Tree, Go Green" Make In India : http://www.makeinindia.com/home