Update index.php

[pravintargaryen]

The compiler is not displaying the $stdout and only the successful compilation message in green. So added same background as $stderr for printing out $stdout variable though I am not sure if you are currently working on this as play.php, play_util.php & sandbox.php does the job pretty well.

Went through some interesting articles on Writing our own C Compilers http://scheme2006.cs.uchicago.edu/11-ghuloum.pdf https://norasandler.com/2017/11/29/Write-a-Compiler.html

[csev]

Play.php was the first thing I wrote - then I wrote the whole autograder and added a bunch of things - I clearly did not bring all that back to play.php. Thanks