Generally, I think you should never choose a technology because of its perceived value on the current job market. You should choose technologies because there's something good about them and you can do great stuff using them. To be able to judge that, you probably should know a few, though.
Also, you don't really switch tech, you just add new ones to your repertoire. If you learn Java now, you can still code PHP, right? My guess would be, that you even write better PHP by learning something else. Not because of PHP in particular, just because knowing more different paradigms helps you understand each one better.
They say the second programming language is the hardest to learn, because it's the first time you have to really grok the difference between language-specific issues (such as typing) vs. API/library-specific (such as the concepts behind a particular framework) and general programming issues (such as big-O). Past that hurdle, it becomes more fun all the time, I think.