Currently, php is the most popular language on the web, popular here meaning sheer numbers. There are number of reasons for this, some of them obvious in retrospect. But then again, a lot of things are obvious in retrospect.
Php is easy to get started with. I think this one most of all is the biggest reason for php's popularity. In its simplest form, php is just a powerful templating language. We can take some html, sprinkle in a couple of php tags here and there, and presto, we have ourselves a dynamic page.
Php is easy to deploy. This almost goes hand in hand with php being easy to get started with. Most web servers are already php enabled, so chances are good that things will "just work" for us out of the box. If they don't, php's popularity virtually guarantees that the solution will be easily found online.
Updates are easy to see. All we have to do is save a file, and reload our browser. As a programmer, I can really appreciate this one, since I'm used to having to restart a process and waiting for it to come back up before I get feedback.
It's easy to find somebody that understands php. I can say this for anything that's really popular currently, so it's not something that's specific to php. What php has going for it here is that there is a low barrier to entry, making it easier for a programmer to quickly learn php without having any previous experience.
If I were to sum up php's downside then, it would have to be its ability to scale in terms of complexity. Most changes are made in an ad-hoc manner, with little thought for the whole. With respect to systems, this is the wrong approach. We want the overall system to have a strong foundation, and have the individual pieces be the ones that take care of their own messes. Php emphasizes getting the individual pieces working, and they often have to know about each other's internals.
For simple systems though, this doesn't matter. If we're just building a doghouse, we don't need to spend the time thinking about having a strong core, because we won't need one. If we just slab things together, it'll hold. We only run into problems when we use this approach to build a skyscraper.
That being said, I think most sites on the web are simple. There's a way to put new data in, a way to control what gets displayed, which could be through a search or not, and maybe a custom workflow for interacting with the data. It's often the case that building up these small pieces from scratch is faster than trying to fit them into the context of a larger system.
Even for more complex systems, there is a benefit to prototyping a naive solution, and seeing where it breaks down. Sometimes it's not where we expect, and this information becomes tremendously valuable when trying to design a more robust solution.
There are attempts to overcome php's shortcomings too. There are many php based frameworks and content management systems out there. Each one of these makes certain separations for you, like MVC, to help ease the burden of maintenance.
There's no denying that php does some things well. After all, millions of web sites can't be wrong, right?