Read about Computer science, data structures, algorithms, Software engineering principles, design patterns, OOP, Functional programming, MVC, Dependency Injection. Many articles available on Hashnode, Quora and Medium. You also may try FreeCodeCamp. Good free online video tutorial series available on Udacity
Learn SQL, MySQL 5.7
Learn unix basics, commands, Apache or nginx, hosts file, make local domain (usually ending with .dev or .local)
Set up your local environment with PHP7, MySQL5.7, Apache/nginx installed, configured (I recommend install all them manually and not using something like WAMP, it's not so hard, even on Windows)
Make a simple own MVC application on plain PHP
Install Composer and learn about it and package management.