Here’s a proper definition given by php official developers
PHP is a popular general-purpose scripting language that is especially suited to web development.
why php ?
Fast, flexible and pragmatic, PHP powers everything from your blog to the most popular websites in the world.
According to the definition of php, php is giant and most popular in the world of programming because of most the written application in use today are built with php.
php is used on websites such facebook, bank of america, chase bank etc. even on some online payment systems like coinpayment and paypal.
Almost 90% of popular content management system uses php including wordpress etc.
why is php free?
php is free and will continue to be free as it though powers so many software and serves as the core programming language for many organisations and company.
php is very easy to learn.
With simply php easy documentation on it official website you will become a guru in writing php codes/logics for websites and web applications.
You can start learning it with just with few guides provided by their developers / contributors via documentation or interactive forum.