Comments form in PHP and Mongodb

Hello i want make form for comments but i dont now how use php code :

It's connection on my database :

<?php
try {
           $connection = new MongoClient();
           $database   = $connection->selectDB('test');
           $collection = $database->selectCollection('articles');
         } Catch(MongoException $e) {
           die("Failed to connect to database ".$e->getMessage());
}
         $cursor = $collection->find();

       ?>

Now my form :

<h3> Add your comment </h3>
              <form action="" method="post">
              <span class="input-label">Name</span>
                   <input type="text" name="comment_name"

                   <br/><br/>
                   <span class="input-label">Email</span>
                   <input type="text" name="comment_email"

                   <br/><br/>
                   <textarea class="input-label" name="comment" rows="5" cols="30"></textarea><br/><br/>
                   <input type="hidden" name="article_id"

                   <input type="submit" name="btn_submit" value="Save"/>
                 </form>