If you plan to be a web developer, you should learn php. Many posters here suggest that you should skip php if you don't like it, but should definitely try learning some new language. Other developers recommend learning php until you can see the beauty in it. But I say, even if php looks ugly to you for the rest of your life, you should learn it, and it should probably be relatively high on your list of things to learn if you want to be a web developer. Here are some common tasks:
Translating an existing site from php
So you're a RoR hotshot? Great! We've been looking for a new webmaster to build our new site. Oh, did I say new? Well, okay, so the customer order forms were implemented three years ago in php by a guy who left the company two years ago. We really need a full on inventory management system, so it would be great if you could implement that, but the order forms need to be recreated or preserved and integrated with the inventory management system.
Adapting an existing technology
What would you have done if you were working for a company that used WordPress as a content management system? You might get to have that thirty-minute bull-session about how ugly the plugin code tends to be, but then what would you do? My company used WordPress, and I had to replace it from scratch. Sure, I had the pleasure of using Django, but we only made this decision after we decided that there was no possible way to get the functionality we needed from a php plugin (and until then we'd adapted it a lot with php plugins).
Translating a site into php
You have a great RoR site. The MVC design clearly respects the boundaries between presentation and logic. And you know that your extensive knowledge of Ruby is going to make maintaining it for any new demand a snap.
Then you get a call from a company interested in white-labeling (rebranding) your product. And they have to host it on some cloud data-center. And it only runs php. Okay, so this scenario has some holes in it, but it's based in something that does happen: you have to produce something for a client who will not or cannot take your input on what language to use.
Freelance work
The next time you need quick money between jobs, you'd be surprised how many posting for freelance web-programmers specify php. I think that organizations willing to debate and choose a web-framework are likely to be willing to pay the overhead to make a new hire. Those who need to get it done right away and not think about it for the time being are happy to specify php.
Of course, if you don't want to be a web developer, don't learn php. Yes, it is always enriching to learn a new language, but if that's your goal, consider Russian.