added 'PHP Sandbox'

- signed-off-by: trimstray <trimstray@gmail.com>

README.md

@@ -275,6 +275,8 @@ for transferring data with URLs.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://www.shellcheck.net/"><b>ShellCheck</b></a> - finds bugs in your shell scripts.<br>
 &nbsp;&nbsp;:small_orange_diamond: <a href="https://jsbin.com/?html,output"><b>jsbin</b></a> - live pastebin for HTML, CSS & JavaScript and more.<br>
 </p>
+&nbsp;&nbsp;:small_orange_diamond: <a href="http://sandbox.onlinephpfunctions.com/"><b>PHP Sandbox</b></a> - test your PHP code with this code tester.<br>
+</p>
 
 ##### :black_small_square: Performance