Added PHP playground (#5103)

Added extendsclass, an online PHP playground

free-programming-playgrounds.md

@@ -162,6 +162,7 @@
 ### PHP
 
 * [Codepad](http://codepad.org)
+* [ExtendsClass](https://extendsclass.com/php.html)
 * [PHPFiddle](http://phpfiddle.org)
 * [PHPTester](http://phptester.net)