PHP, or Hypertext Processor, as it full name goes, is a new generation scripting language that has extensive use in the web community. It is an interpreted and weakly typed language that was built solely for powering the web.
PHP is employed to "talk" to the server, i.e. accept input/feedback from the front end, proccess it and then work on the result on the server side.
PHP is also used to manipulate databases, MySQL being the most popular combination.
PHP is the driving force behind Wordpress, a Content Management system that enables non-coders and coders to rapidly build their own websites without too much stress. As at 2018, Wordpress powers over 30% of the world's websites.
Popular frameworks include: Lavarel, Cake, etc.
Beginners can get started with these links: https://www.w3schools.com/php/, https://www.youtube.com/watch?v=7TF00hJI78Y