Php is one of the widely used internet language these days. The popularity of php is increasing day by day. Every second website u see on internet is made with php. Php has many advantage over other internet languages. Some of the reason of php popularity are:-

1. Easy to learn: Php is easy to learn and a new comer gets his hand wet and become expert in 1 month if he works hard.
2. Cheaper Hosting: The hosting of php is cheaper than .net hosting and java hosting. And even there are free hosting available for php.
3. Many Frameworks: I think php is the only language which has large number of frameworks. The thing to note is that all the frameworks have same logic behind. So if you master in one framework then grabbing other framework is a child play.
4. Open Source: Php is also open source that mean you can extend and change the source code while keeping in mind the copyright.

So there are many other advantages of php also but popular one is these four i have discussed.