The history of PHP is featured on the PHP website PHP: History of PHP. The gist is PHP was created in 1994 by Rasmus Lerdorf. He wrote PHP in the C Programming Language.
His original reason for creating it was more for personal purposes hence why it was called “Personal Home Page Tools”
Over time the language was fine tuned and was transformed into the PHP that we use today.
PHP, is the most widely used server side programming language used to power some of the largest websites online today and usage is actually increasing. Usage Statistics and Market Share of Server-side Programming Languages for Websites, July 2016
PHP also helps you connect to a database like MySQL which makes it very useful.
PHP, is also one of the most well documented server side programming languages and you will find an endless amount of tutorials out there to learn how to use it. Choose your tutorials wisely as some may not each you proper coding standards.
Here are my recommendations:
The list above will get you started on the right track.