Back when all you had for choices were Perl, Java, and C++ it was easy to see why PHP got big. However there are issues with starting out on PHP now: 1) It's really only meant for web development. 2) There are other languages now that are simple to learn, flexible (more than just for web apps), and powerful (Python, Ruby, and now even Javascript) Unless PHP has another major evolution, like its move to OO; (or unless…
"it was easy to see why PHP got big"
It's easy to see why it's staying big:
1. cheap shared hosting to get started 2. most hosts offer one-click installation of any sort of application people need on the web (blogs, shopping carts, calendars, social networking, file hosting, etc) 3. 15+ years of knowledge about what works and what doesn't when attempting to scale PHP to large needs. 4. ubiquity which keeps a large pool of reasonably priced talent available, which perpetuates the cycle.
When there's a competitive language that can be placed in a directory as FOO.abc, I can hit it as /FOO.abc and have it execute, with html content type sent by default, it'll have a running jump at dethroning PHP.
The barrier to entry is the lowest out there, but the heights it can scale to are facebook-level and beyond.