Gosh, when are these "death of x" things going to stop...this is just fear-mongering.
Nothing against the OP, he probably read it somewhere but there is no reason to ever worry about things like that. I remember back in the day when Steve Jobs killed OpenDoc
The OP's question reminds me of the "question" asked by that guy to Steve Jobs. Or rather to be more specific it reminds me of another instance, now that I'm thinking of Apple, the flash issue. Everybody was attacking Apple for not including Flash in iOS and how much of a drawback it is not having flash and at that time they, or rather Jobs wrote thoughts on flash and a few years later Adobe scrapped mobile flash as well.
You have to make a choice on which platforms to use, because some platforms do and some don't have a future.
Regarding your question on the death of PHP and being worried about it. PHP is gonna stay around for a while, but even if it doesn't there is ABSOLUTELY no reason to be worried if it dies. Because it won't die, it will be replaced by something that is more capable, more efficient but it won't just disappear. So you should NEVER, EVER worry about things like this.
My last point is about your thoughts on the language of choice, PHP vs App languages as you put it. Again, there is no reason to worry here either, I'm not a programer myself but I Know a lot of them and if you're a good programmer adopting a new language isn't more than a few months of work, if not less. Most people make the mistake of thinking that there is something incredibly complicated about the language syntax that has to be learned, no...programming is so hard because you actually have to re-wire the way you think, that's the hard part...learning the syntax is something that most of us can do in a month or two.
Hope that this satisfies your curiosity and erases your fears.