As a college CS student it doesn't matter much if you have PHP or not. At this stage, you will be looking to build a solid foundation of creating software systems, regardless of language. Stick with what interests you, as that will help keep you motivated.
What scenario will PHP be useful in Java project? Hmm commonly changed dynamic files such that changing the JSP/ re-deploy web app is overkill; such files may be better off developing in PHP and kept in the Apache HTTP server. Maybe XML data files and schemas?
Depending on the actual project, most of the time the web server (usually Apache HTTP) will forward to the Tomcat or whatever Java app server through mod_jk or the equivalent and once in the Java app server, everything is java.
I am a Zend Certified Engineer and I... do not like PHP. It's a good language. It is very easy to learn the basics and get something that works. And, ironically, it's a problem. The problem is, many people learn those basics, get things working, and consider themselves "PHP programmers". I have seen countless terrible codes during my professional work in PHP. This is what led me to quit PHP. It was enough to me to deal with terrible code written by "professionals".
IMO: If you love purity in code and architectural excellence, than PHP may be not for you. If you want just to get things done and get paid, PHP is okay.