Is this a school assignment or is there some other reason why you want to do this? If you are going to put a lot of time and energy into building a site, then you should focus it on something that interests you. It could be music, art, computer programming, astronomy or any one of a million topics. It won't be very satisfying if you are building something that you think other people will be interested in but you aren't.
Thus, if you want some useful ideas, it would be helpful if you identified: 1. Why are you doing it (what is the objective or the definition of success)?
2. What are some categories that interest you?
3. What is your skill level in PHP and other associated technologies? Is this a simple site so you can learn PHP or is it intended to be a bleeding edge, high tech site?
If your intent is mainly to make money, then you need to do some research on Internet Marketing and Search Engine Optimization. The PHP is secondary. You can easily build a site with Wordpress or some other CMS but you will probably want to find a "niche" where the competition isn't too strong. There are all kinds of websites that will give you advice on this and Forums like DaniWeb's Internet Marketing Forums where you can get info and ask questions. A course that you might find useful is here (it isn't free). It's about becoming an Amazon Affiliate and finding niches for products that you can market. There may be other ways to build a site and make money, but this is the simplest and most common approach. Google is becoming much harder on sites that have little useful content and are just designed as Affiliate sites. Thus, if you go down this path, you still need to develop some good content about your website topic.
If your real intent is to build something innovative and cutting edge (the next Facebook!) AND make money, then you can probably come up with some ideas but you could spend a huge amount of time building something that goes no where. There are many failures for every success. You may want to undertake a project like this just for the experience and the possibility that it might make money but it depends on your priorities.
What's your interest? Start off with something in which you have an interest and can monetise. This will probably mean updating the site regularly to keep content fresh, so it should be something that interests you and something you know A LOT about. Good, unique ideas are hard to come by, cheap copies of existing sites are 2-a-penny. If you're going to make serious money, I imagine that you'll need to get a huge number of hits (unless you sell a few units for huge profit), so your site needs to be scalable. As mentioned by chris, SEO and IM will be vitally important. Getting these right may incur a cost, possibly substantial, depending on the market.
//just re-read chris' reply.
The language you choose for the site is secondary. IMO, the idea/implementation and realisation are the main issues. You could make it with ASP.NET, java, coldfusion, static HTML/perl, ROR, php. I don't think anybody would care.