It's easy to write in a way which is stupid.
You can write good php and you can write cool stuff with it. It is easy to non-technical folk to pickup and write a little bit of.
It's also amazingly easy to:
1) Write code which is broken and stupid (woops, left error_reporting(E_ALL) on! silly me. dumped all the passwords out into the website. woops. stabs designer)
2) Write code which is clever, but different from the coding style of every other piece of code in the project (>_> look, my js is so clever, its using closures for everything and events and... oh, you guys weren't doing that? um... ok. Well, now we have some piece of code that only one person knows how to understand stabs self important javascript ninja with spoon).
3) Introduce bugs.
Managing your developers is really the key. If you can keep your php programmers in line, and keep bad code out of your project, php isn't that bad.
...but its that much harder to do when twice as many people are working on the code, and some of them belong to another team.