Added a learning material (#4279)

Added a new PHP tutorial

free-courses-en.md

@@ -419,6 +419,7 @@
 * [Learn how to program: PHP](https://www.learnhowtoprogram.com/php) - Epicodus Inc.
 * [PHP & MySQL Tutorial](https://www.youtube.com/playlist?list=PL_RGaFnxSHWrjkpK2zD4TWKWMWVfeYK-b) - The Bad Tutorials (2015)
 * [PHP Programming Language Tutorial - Full Course](https://www.youtube.com/watch?v=OK_JCtrrv-c) - freeCodeCamp.org (2018)
+* [PHP Tutorial](https://www.w3schools.com/php) - W3Schools
 
 
 ### Python