You can make a website about anything, especially academically. Make a page about cats, and how different cats make dogs bark differently. You are proving functionality, concepts, and reiterating all the things you have learned in your class. Content, for the most part, is secondary.
PHP is about making frames, and filling those frames with content/data (and fetching/saving it securely, if done right). Sure, you could go beyond that, but in it's basic sense, that's all it is. Show you understand how to do those things, and base your content on a hobby or interest of yours.
Obviously, you enjoy programming or have an interest in it - make a website about it!
Have the world's biggest beany baby collection? Tada! Website!
As stated above, depending on your education level and what is expected, you can always add functionality.
Beginner - just make it display/show info. Maybe make a web 2.0 interface so people can comment on all those cutesy beany babies after a secure login.
Advanced - why not sell them?
Super - Mega - Advanced - make software that will let people talk live to the beany babies in a secure fashion, and include handlers for pay-per-minute conversations.
My first academic website was a "share your concert experience" website, based on web 2.0 social interaction. It was awful graphically, and its layout left a lot to be desired (I had 2 weeks to pump out a full site from scratch, by hand. No JS, or other tools than PHP/MySQL and HTML/CSS). However, the idea worked, I showed concepts of self/group moderation, logins, use of session handlers/cookies, etc... It was about showing I understood the material, and was able to visualize what I had learned and apply it. Your content is not important, unless you plan on making the site live. Then, it might be worth your while to make it interesting.
Remember, your academic career is about building up your "tool-box" so when you go into the working world you don't necessarily have to do it again. It's all done for you, and you can build off what you have in place. Or, get familliar with frame works, and do it off someone elses hard work instead ;)