What next?

This has only been a cursory introduction to the PHP language and ecosystem. Here are some ideas for things to try next, in no particular order:

If you enjoyed this tutorial, or found it useful please let me know and share it. If you think it could be improved, let me know that too. You can find me on twitter. You can also find the source code of this website on GitHub.
Take a break from the screen - there's good evidence that you remember more of what you've learned shortly before or after a break.
Write your own PHP program, or dive into a PHP codebase and make some improvement.
Browse the official PHP Docs. There's a lot in the language that we haven't covered.
If you find a symbol that you don't understand in PHP code, look it up in the What does this symbol mean in PHP? Stack Overflow reference page.
Learn about PDO in depth with (The only proper) PDO tutorial.
Read the Twig Documentation and learn how to make more sophisticated templates.
Watch the talk Aggressive PHP Quality Assurance in 2019 by Marco Pivetta, to learn that "PHP is a horrible language that allows you to do too many things", and how to use the tools that will tell you not to do the bad things. In this tutorial I've tried to show only the good things you can do in PHP.
Learn how to use a PHP Framework or CMS. I would personally recommend Symfony, and the Symfony Getting Started guide.

You could also try the Laravel Quickstart Guide, or a CMS - perhaps First Steps with WordPress or the Drupal 8 Documentation.
Read PHP: The Right Way. This covers a lot more topics than we've been able to include here, including a lot of the history of how PHP got to be the language it is today.
Browse PHP packages on Packagist. This is where Composer looks by default when you tell it to install a library or tool.
Subscribe to PHP Weekly News for interesting links every Thursday.
Subscribe to PHP Annotated Monthly from the makers of PhpStorm, for interesting links every month.
And finally, read Accelerate: The Science of Lean Software and DevOps: Building and Scaling High Performing Technology Organizations, by Nicole Forsgren, Jez Humble and Gene Kim. This isn't about PHP, but I think it's perhaps the most important book for anyone making software in an organisation to read.

Accelerate describes the capabilities that organisations need to develop and maintain to perform well with software. You can also read about those capabilities in the briefer and slightly more up-to date form of the 2019 Accelerate State of DevOps Report.