+ PHP is universally supported, virtually out-of-the-box, on virtually every architecture, distribution, web server, hosting company, etc etc etc you could name.
+ The deployment story for PHP apps is better than it is for e.g. Rails apps, particularly if you have, um, people who are not primarily technical in charge of it. For example, if you can follow instructions, you can install WordPress, and it will take you about five minutes. Given a functioning Rails application and told to put it on the Internet, and assuming you're a competent systems engineer, allocating an afternoon to the project is not a bad idea at all. (Unless you already have made the upfront investment in e.g. Chef/Puppet/etc scripts to configure everything your app will need to get running. I have a project where I haven't, but I've got pretty good documentation on what needs to be done to bring up the whole environment, and it takes me 2 to 3 hours to do.)
+ Programmers are rare. Programmers close to your are rarer. Programmers close to you willing to work for what you're willing to pay are rarer still. Yadda yadda. All of this is true about PHP programmers but they're about an order of magnitude or two more common than e.g. Rails programmers.
+ Even if we concede that PHP is a language comparable to duct tape and bubblegum, I've got to point out that a lot of the world just needs duct tape and bubblegum. There is a web service running at my old day job, where hitting a URL is supposed to tell you when the most current backup of the database was taken, and that's it. It is written in PHP and was coded, tested, and deployed live in approximately as much time as writing this comment took. Rails -- which I love! -- is not a very good hammer to hit that nail with. (I've got an app or two on Sinatra internally, and while I like them, they feel inferior in this case to "We already have Apache up. Copy/paste one file and forget this was ever a task.")