agree; second major reason: facebook

Facebook making effective use of PHP as the basis for a scalable and reliable service has legitimized it as a language that can handle modern issues of scalability and code re-use almost as well as any others so long as discipline and avoidance of the less desirable aspects of the language are adhered to ... much the same way Javascript is witnessing a rebirth as "Javascript: The Good Parts" combined with the popularity of node.js is elevating it to leading language status.

Yes, Facebook definetely helped PHP in getting credibility as a solution for serious sites with large audiences.

It is just hard to measure how much Facebook adoption of PHP made a difference in PHP popularity.