Added comments in php scripts for documentation

backend/compile.php

 <?php
 
+/*!
+This script compiles a given code file (except python code files) for a user using the compiler correponding to the language
+of the code file, using system() function.
+*/
+
 header("Access-Control-Allow-Origin: *");
 header('Access-Control-Allow-Credentials: true');
 header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE");

backend/database.php

 <?php
+
+/*!
+This is the default script to connect with the MySQL server in the backend. The login and registration php scripts use this.
+
+*/
     header("Access-Control-Allow-Origin: *");
     header('Access-Control-Allow-Credentials: true');
     header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE, OPTIONS");

backend/dir_tree.php

 <?php
+/*!
+This script returns the directory tree of a user's workspace, given the username. It returns a nested array in JSON form, after
+using a recursive function called dirToArray()
+*/
 
 header("Access-Control-Allow-Origin: *");
 header('Access-Control-Allow-Credentials: true');

backend/execute.php

 <?php
 
+/*!
+This script uses the username, name of the code file, language of the code file and the input data (test case) and executes the code,
+(given that it has already been compiled) using system() function, and returns the output. If the file hasn't been compiled or a runtime error occurs, then
+the script raises an exception
+*/
+
 header("Access-Control-Allow-Origin: *");
 header('Access-Control-Allow-Credentials: true');
 header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE");

backend/filedelete.php

 <?php
 
+/*!
+This script deletes a given code file, or a directory in the user's workspace, given its path.
+*/
+
 include_once("database.php");
 $postData = file_get_contents("php://input");
 

backend/filesave.php

 <?php
 
+/*!
+This script saves a given code file for a user, given it's path.
+*/
+
 include_once("database.php");
 $postData = file_get_contents("php://input");
 

backend/get_file.php

 <?php
 
+/*!
+This script uses the username and file path in the user's workspace to return the contents in the requested file (code file).
+If the file path is not found on the server, a 404 HTTP error is raised.
+*/
+
 header("Access-Control-Allow-Origin: *");
 header('Access-Control-Allow-Credentials: true');
 header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE");

backend/list_files.php

 <?php
 
+/*!
+This script returns the code files present in a user's workspace, but doesn't account for directories and sub-directories.
+This script is obselete because of the script dir_tree.php, which returns the directory tree of the user's workspace.
+*/
+
 header("Access-Control-Allow-Origin: *");
 header('Access-Control-Allow-Credentials: true');
 header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE");

backend/login.php

 <?php
+
+/*!
+This script is for user login using username and password. Here, we authenticate user credentials with  MySQL server.
+*/
+
 include_once("database.php");
 $postData = file_get_contents("php://input");
 $request = json_decode($postData);

backend/questions/get_questions.php

 <?php
 
+/*!
+This script checks the questions present in the MySQL database, and sends the question info (title, problem statement, 
+start and end time, and the author's username) to the frontend. 
+*/
+
 header("Access-Control-Allow-Origin: *");
 header('Access-Control-Allow-Credentials: true');
 header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE, OPTIONS");

backend/questions/question_save.php

 <?php
 
+/*!
+This script saves a question in the MySQL database after a user creates it. It uses parameters like, author's username,
+problem statement, testcases, start and end time, etc. 
+*/
 header("Access-Control-Allow-Origin: *");
 header('Access-Control-Allow-Credentials: true');
 header("Access-Control-Allow-Methods: PUT, GET, POST, DELETE, OPTIONS");

backend/register.php

 <?php
+
+/*!
+This script registers a user using a username, password, name, and email. This script communicates with the MySQL server to save
+user data.
+*/
+
 include_once("database.php");
 $postData = file_get_contents("php://input");