freeCodeCamp's The PHP Handbook teaches PHP fundamentals, object-oriented programming, and working with databases. The entire course is free on freeCodeCamp.org in video and written formats. After finishing, you can write PHP scripts and understand how PHP powers dynamic websites. PHP runs on about 77% of websites with a known backend, making it one of the most widely deployed languages. The main limitation: older language with some quirky design decisions. Modern frameworks (Laravel, Symfony) are more enjoyable to use than raw PHP, so pair this with Laravel to see PHP's full potential.
PHP is used in 77.5% of all websites with a known server-side programming language according to W3Techs (source: w3techs.com/technologies/details/pl-php).