If you are in a learning mood, I'd say go with rails.
If the point is to build the video hosting project, and you are productive using PHP, then that's probably the best option.
If the point is to increase the breadth of your development skills while building the video hosting project, choosing a new language like Ruby might make sense.
If you've identified a specific technical challenge in what you want to do that makes PHP problematic, investigating alternatives (including different languages, though that's probably not the first thing to look at) for that particular problem also makes sense.
Then I thought to myself. I wonder if Zuckerberg back in 2004 asked himself that.
If Facebook is capable of running the most popular website in the world successfully on PHP, then it'll probably be good enough for me!
My vote goes for PHP. Stick with what you know.
Rails is great, not for everything, but it does make standing up a new site pretty darn easy. Personally, I won't touch PHP again if I have my choice.
So, if you're not in a big rush, I'd learn a better language than PHP.