Anyway, is there any reason to learn PHP & JavaScript? I'm going to eventually learn them both, but it seems like they do the same stuff. What are the strengths/weaknesses of each?
They're very different. JavaScript is primarily a client-side language - it runs on the user's computer in the user's browser (it can also run elsewhere, but that's not as common or important). What's important is that it is the 
only well-supported client-side web language. If you ever want to run any code in a browser, you'll have to either learn JavaScript or use a framework that translates another language to Javascript (e.g. http://code.google.com/webtoolkit/). Unless you go with Silverlight. Even Flash is powered by a variant of JS, called ActionScript.
PHP, on the other hand, is a server-side language. It's decent and very common, but far from unique. Any other language can do the same work, and often does.