PHP is indeed still relevant, and it will remain so for next few years or till the Apocalypse! Reason? As Cliff said, it is really easy to get started and you already have the libraries you need to get your application up and running, at an unbelievable rate(I mean, time)! Let say, for an example, I can build a simple CRUD(hypothetically speaking) project faster in PHP than Java, not that I'm bad at Java, but the ease of doing things. And the hosting! It is cheaper and options are endless. To tell you how cheap it is, I've once bargained a vendor to give me hosting for 1$ a month and he gave!!! Because my bandwidth is not a lot and project is not big either.
So unless something trumps these advantages of PHP, it will stay. And I don't see any new-comer going to shake things for PHP, because it already spread out and going to stay strong till the end!