Sending/receiving files over port 80

[eluser]comex[/eluser]
Hello (please excuse the confusing title),

Background
I have a web server hosting a webpage (powered by CodeIgniter) where users input information that is to be mirrored to a database on a different computer. The other computer hosts an application where users locally submit the same type of information as they are able to submit through the web page. Only difference is that information submitted through the web page has to be approved by someone on the "local side."

Problem
I want to let the other computer to request the changes that have happened in the web servers database and then to have the web server respond with an XML-file containing the changes. And then for the other computer to send an XML-file with whatever changes has happened on that side( for example the approved entries on the web page.)
I'd prefer to keep this on port 80 in order to limit the amount of open ports on the web server(which is the only computer connected to the internet.) Also, this process has to be able to be scripted somehow so that it can be done automatically.

I haven't been able to find any pre-made solution, but I'm also quite inexperienced with web-programming so I don't really know what to look for. Also excuse me if the question is a bit hard to understand I'm not able to quite wrap my head around the problem.

Thank you

Edit
I should note that the amount of users on the web site will be quite low, an estimate is at max 20 users per day.