According to the StackOverflow developer survey of 2019 over 26% of developers use it and its said to be used in almost 80% of websites today.
PHP has come a long way from writing flat PHP pages to having decent object oriented features, frameworks, content management systems and command line tools.
If I may recommend a few;
Tools like composer have made package management a breeze.
I would recommend starting with PHP: The Right Way because there are a lot of outdated material out there.
Start with the latest stable version PHP 8.0 at the time of writing. Anything before 7.2 is really should be updated and pose a security risk to keep around.