Trends in Programming Language Book Sales vs. Jobs
For the graphs below, I was most curious about PHP, MySQL, Java, and ASP.NET.
Found this graph of O’Reilly book sales:

And then did this search for job trends:

Java, as suspected, is clearly the winner in both categories. What I thought was interesting was that, in terms of book sales, PHP sales are more steady than Java books and are currently about half of what Java sales are. But if you look at job trends, Java is dominantly and consistently approximately four times as more prevalent. So based on these fairly off-the-cuff metrics, I would guess that there’s people out there learning PHP for their own purposes, their own startups (ie. digg is PHP), etc.
Make of it what you will. I like the numbers.

Well a lot of things should be considered. There are a ton of PHP open source projects, and it’s a good language to know if you plan on maintaining your own dynamic website or a blog.
From what I hear, it’s easier for students to jump into PHP than Java (though this is based on just a couple of personal accounts)