And welcome to WebmaisterPro. The basics of PHP are very much straightforward, though in the last few years PHP have really changed a lot and have become a "real" programing language.
There are many online resources related to PHP and probably the PHP community is the biggest compared to others. Good start would be https://php.net/
actually you can always use the official website as reference.
Don't forget that to write decent PHP you need to be proficient in HTML, as well as CSS and of course MySQL database.
"PHP for Absolute Beginners" book will be an excellent start.