I'm going to apologize in advance if any of this seems too stern or rude.
You seriously don't have the correct mindset about this. You are thinking in a way that a lot of other people share, but it's not practical. In the real world you have to learn new technologies and languages consistently. Every day something new comes out or gets improved that makes doing a difficult task easier, and people/companies that can adapt and not be left behind do well.
If I were in your position (which I guess I kind of am, I'm also college-age but I've spent just under half my life doing some level of professional programming) I would be learning EVERYTHING I can or at least everything mainstream I can so that if I come by a cool piece of code online or at work I can tell exactly what it does.
It's true that PHP isn't as a strictly written language as some of the other languages (by this I don't mean syntax, I mean that doing two analogous operations have completely different names/attribute orders in the standard library) but it's still used by tons and tons of people. PHP isn't dying. I hate the term "dying language" because there's always something else out there that's good for that each specific language.
I'm assuming that Flash is also "dying" in your POV but just today I had to whip up a quick SWF to allow copying something to a user's clipboard because it was the simplest and quickest way to allow one-click clipboard access that I could think of and I have spent time learning Flash/AS in the past so I had that knowledge.
Now, when you say:
it seems like it might be a waste to spend time at Facebook using PHP if the rest of the tech and start-up world has moved on
I have no idea what you mean. Do you mean that Facebook not restructuring their entire codebase to a cooler and more popular language is a waste of Facebook engineers time? Or do you mean that you learning PHP to work at Facebook would be a waste of your time? I don't understand how either of these two statement could be argued so if you meant something else I'd love a clarification.
There is absolutely nothing that Facebook can not do with PHP that they could do with Python or Ruby. Any business logic can be written in a myriad of ways. And I believe to be a strong candidate for a "wow" employee after you graduate you should be able to drop head first into any codebase and at the very least quickly get a simple grasp of what's going on.
If Facebook gave you an internship today would you reject it because you've heard bad things about PHP?
Sorry about the rant, but I've spent a lot of time working to hire other developers and it really irks me when I hear something like "oh so and so's a dying language, it's useless to learn it" because it shows that you don't have the drive and love of technology that my ideal person would.