This is just my own opinion (and anything else would make no sense, because "importance" is a purely social term, any "objective" answer is impossible):—SA
There are many weak or lazy programmers, and workers in general, who don't want to learn much, and do not understand that if you want to get something good, it's never too easy. That's why some sloppy, small and rudimentary tools like PHP become sometimes very popular. There is a nothing really good about PHP, except just few things: it is extremely easy to learn to the extent where you can start making some application, and also it is quite light-weight. At the same time, it is rudimentary, don't follow strict programming principles, with no fresh or elegant ideas, a lot of an ad hoc tool. There is nothing special about this language, not a single interesting feature, but a lot of really obsolete techniques like modularity based in inclusion. And nothing specially attractive.
But once the language got some popularity in some quick and dirty work, another powerful factor came into play: unlike other server-side technologies, it comes by default even with the very cheapest (or even free-of-charge) hosting plans. This economic factor makes PHP attractive: sometimes you want to have a minimally supported Web site at really low cost, but you cannot create it without server-side programming at all: you need at least some communication page, or a block engine, or a minimal chat, or stuff like that. For such functionality, you could tolerate PHP for a while, until you complete the job. Well, it could be worse. Actually, many create really serious big work with PHP, not because it's good, but due to its low cost and availability of many free components written in it.
PHP is a server language this means that the statements written in the language are executed at the server by a PHP engine and eventually translated to the client (web browser). There are other languages who played the same role and many of these provide better conditions for the programmer in terms of debugging, legibility, readability, etc. PHP is extremely dynamic this makes it difficult to test and find errors.