Auto submit a form that launches a PHP scripts using Ajax/Javascript

I;d like this simple "chatting" site to work, but the messages update only if you hit "run php" to submit the form.  I don't know Ajax that well. But I tried to  get  some simple JavaScript to work, so that  the form is submitted, say, every few seconds by itself. But no luck. From this link and if you view the source, you can clearly see what I am trying to do. The  Java/Ajax section was from some random place, but it wasn't as simple as I expected.

http://cs330.blueearthconnections.org/Ze/Spring2016/page_with_use_php.html

If you follow this link and type something in the slot and hit "run php", you will see what I am trying to accomplish but without luck yet.
Please offer some suggestions, sample code or fixes will be highly appreciated.

Thanks in advance