PHP's public image problem is mostly due to the direction from which you can approach it. You can be the maintainer of a static HTML page who knows nothing at all about programming, and someone can help you dip your toe into PHP by showing you some simple tricks, and suddenly you have the ability to start adding code here and there, copy/paste style, to your content. The results are predictable. And for better or for worse, the language has been given some ill-advised features in order to cater to this type of user.
And you can approach it in the opposite direction, as well: as a programmer who knows what he's doing, you need to choose a language and a platform to run your app on, and PHP is ubiquitous and perfectly capable, and you know which ill-advised language features to avoid, so it's a reasonable choice.
The difference between PHP and most other languages for building web sites is that with most other languages, you simply can't follow the first path. Web servers that let you just drop snippets of ruby into HTML are not everywhere (I assume that exists in some ill-advised apache module, but I haven't personally seen it). You need to know a significant amount up front to get your code running, so the "never even considered programming before" portion of the population is naturally suppressed. It doesn't mean they don't exist or that somehow making a language more difficult to use automatically makes its users better, it just means that a certain type of amateur user is naturally suppressed, so there is proportionally fewer of them.
Note the similarities: ActionScript/Flash is actually a pretty cool piece of technology [let's ignore their poor track record with regard to security holes in the runtime, since that's an orthogonal issue]. And yet it's terrible to program in for a seasoned developer because the community is so chock full of artists and amateurs who just dipped their toe in to add some minimal interactivity to their drawings that you often have a hard time finding docs that are written above the copy/paster level. And well-written ActionScript code is a relative rarity as a result. I'm sure that on at least some level, any seasoned dev who primarily uses PHP can relate.
