There are lots of reasons.
* PHP attracts idiots. Partly that's because you don't need to know anything to make simple PHP. So, in the large pool of PHP programmers, there are a ton of idiots. And those idiots produce a lot of verbose code that gets used because it does something. Ruby, Python, Perl, etc. all put up a larger barrier because they don't just come with a session handler or whatnot built in. And that weeds out more of the idiots. Doesn't really have to do with PHP the language, but it is nonetheless significant.
* PHP has a bad type system that leads to errors. There, I said it. The fact that 0 == "string1" and 0 == "string2", but "string1" != "string2" is unacceptably bad.
* PHP has no namespaces. Yeah, they're planned for 5.3, but the implementation is just ugly. This makes it more likely to botch something in a larger project.
* Which leads me to my next point: include/require. In PHP, it's decently common to say require('some_file.php'); and just grab a huge hunk of code. Eww! That code could be doing anything - adding in variables, executing code, etc. While it's true that other languages can do that as well, that's generally not the case - other languages are more in favor of importing objects or functions. Partly it's a language thing, partly it's a culture thing.
* Magic! So, how does mysql_query() know which database to connect to when you don't pass in a database connection? I mean, I'm guessing they're either using a global or if it's in C a static external variable, but PHP is littered with this crap. It's not necessarily bad, but it's magical.
* Functions! So, PHP now has cool object oriented stuff. Still, it doesn't use it at all. I don't object to strlen($var) that much when compared to $var->len(), but there are other cases where it's just crappy. So, PHP has a function strpos() which will return the position of the first occurrence of a substring within a string. Quick! In what order do you put the variables? In this case, it's string to be searched, search string. What about preg_match()? Oh, there you put it in the reverse order! And really, when writing functions, there is no "proper" order, but if you're doing $string.preg_match('/^apple$/'), there's a clear order.
* Looking more at preg_match(). What does it return? It doesn't return a match, it returns a count of the matches it finds. If you want the matches, you need to pass in a variable that it will take a pointer to as an argument so that it can place the matches in there. Bleh! How would this be done in a nice OO-way? $var->preg_match('/^apple$/').count() would get you the count and otherwise you get the match returned. Easy in a way that requires less memorization of weirdness and doesn't feel so wrong. And crappy things like that are done throughout PHP where things are weird, but end up
* php.ini. Terrible idea. PHP allows you to change very important things install-wide, directory-wide, app-wide, ick-wide! It's not just magic quotes. It's more that you never quite know what you're deploying to in a bad way.
Finally, it's that PHP is an older-style language without the cool things that newer languages can do. PHP isn't bad per-se, but it isn't good either. The biggest thing it has going for it is that it's easy on shared hosting. With VPSs coming down in price to the point that they don't represent a huge premium over shared hosting, that's become less of an attraction. PHP is fine, but there's a decent amount to get annoyed with when creating a larger application.