update php resource (#6357)

* update php resource

An online book for learning PHP

* fix lint errors

* add in process

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

Co-authored-by: Eric Hellman <eric@hellman.net>
Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -1701,6 +1701,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 
 * [Clean Code PHP](https://github.com/jupeter/clean-code-php)
 * [PHP 5 Power Programming](http://www.informit.com/content/images/013147149X/downloads/013147149X_book.pdf) (PDF)
+* [PHP Apprentice](https://phpapprentice.com) - Andrew Davis *(:construction: in process)*
 * [PHP Best Practices](https://phpbestpractices.org)
 * [PHP Documentor - Documentation](https://docs.phpdoc.org) - phpdoc.org
 * [PHP Internals Book](http://www.phpinternalsbook.com)