Compared to many of the alternatives at the time, php was easy. Until you are trying to make a large application that needs lots of abstraction and careful architecture, it stays easy. It is very easy to get something to work quickly in php, it's just hard to maintain a large project in it.

And it is much more accessible than almost anything else when it comes to web programming. Compared to rails even, php is a snap to set up and use. Rails has the disadvantage of being a framework, which means more learning up front. Php has the disadvantage of being (until v5) almost impossible to organize coherently, and so becomes hard to upgrade later.

Php is easy; until you've gone too far to turn back. :(

That's why it's dangerous.