what is php and its application, what is the best way for get trained?

i would like to learn php. i would go into the IT section and i really love programming. i have good knowledge in java, python, C/C++, C# and did several student project and freelance project in my area.

now i am wondering what the application of PHP is (actually i am zero about it) what is the main and practical learn of training for this language ?

i am sorry that my question is very general but any respond from you is welcomed.

thanks :)