Why I believe I'm qualified to give an answer? In my late teens when I absolutely had to a have a job and bring money home I went for enterprise programming gig because they'd have me and, more importantly, pay me outright instead of a systems programming job where I was told "please come back once you've learned X". Over the years I became a sought after specialist in my professional area and I love it, on the other hand I'd have to take a serious pay cut and still learn X properly and then still would have to look very hard for a systems programming job to make an entry, because there are not that many of them.
I'd seen this pattern repeat itself many times over with talented people aspiring to be systems or games programmers becoming highly successful business analysts, quants or system administrators because of the first paid job they took. There is nothing wrong with specialising in any other type of programming or becoming analyst if that's what you eventually became good at.
There are many more web development jobs out there than system programming jobs and until you earn yourself a name in systems programming finding a well paid systems programming gig will be hard.
Breaking into low level programming is unlikely to become much easier once you had a PHP job. Most of PHP web development out there is just not hard enough to demonstrate to a potential employer that you've got what it takes to hack compilers and operating systems.
This is not to say that PHP jobs are dead easy (take Facebook as an example), they require a lot of brain power but complexity is not technical, the complexity is in analysing intricate and often arbitrary business problems, then applying fairly basic coding to create a solution.
And this is probably the wrong type of complexity to keep your brain occupied. A job like that might influence what you think about most of the time and the thing on top of your mind might become "the best way of generating customer mailings based on a range of pdf templates without buying expensive third-party components" instead of "building C compiler with tail call optimisation" or "designing a better Java".
The emphasis in run-of-the-mill commercial web development is on delivering code what is just good enough in most scenarios as fast and as cheaply as possible. In systems programming on the other hand a program must behave predictably in all conceivable scenarios. This results in rather different working practises.
None of what I said is cast in stone, career and life are not about certainties, but little things do count at the end.
Here are some questions to ask oneself:
Have you looked for a low level programming job hard enough?
Have you looked for a systems programming job at all or are you tempted by first web job that presented itself without much effort on your part?
Is there any other job you could take that is not going to displace systems programming from the top of your mind?