check_urls=free-programming-books-langs.md Added new PHP language book (#6869)

## What does this PR do?
Add resource: New PHP language book

## For resources
An Introduction to the PHP Programming Language

### Why is this valuable (or not)?
17 in depth chapters covers the basics of PHP programming covering basics and advanced concepts like OOP.

### How do we know it's really free?
Free and published as Creative Commons Attribution-ShareAlike 4.0 as per book's license

### For book lists, is it a book? For course lists, is it a course? etc.
Book

## Checklist:
- [x] Read our [contributing guidelines](https://github.com/EbookFoundation/free-programming-books/blob/main/docs/CONTRIBUTING.md)
- [x] Search for duplicates.
- [x] Include author(s) and platform where appropriate.
- [x] Put lists in alphabetical order, correct spacing.
- [x] Add needed indications (PDF, access notes, under construction)

## Follow-up

- Check the status of GitHub Actions and resolve any reported warnings!

books/free-programming-books-langs.md

@@ -1719,6 +1719,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 
 ### PHP
 
+* [An Introduction to the PHP Programming Language](https://codeahoy.com/learn/php/toc/) - CodeAhoy (HTML)
 * [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)*