How to change a web application that now needs to become OLTP and OLAP application?

Previously, I did a simple web application for an enterprise client using PHP and MySQL.

After some time, it has grown to a point where I realized after talking with people here that I need to treat it as two separate apps OLTP and OLAP.

I have tentatively decided that

1. the OLTP will still continue to run using the current PHP codebase and MySQL


2. the OLAP will use PostgreSQL


3. both should appear as a single app to the end user. This is to avoid inconsistencies and too many logins.


4. the OLAP should help with sending out scheduled reports via email

I want to avoid a "big bang" approach. At the same time, I don't want to do the OLAP as an isolated app.

I want to slowly evolve to the point where transaction processing and the analytic processing functions are modular and consistent.

How do I approach this and bearing in mind, I currently have only 1 physical server that acts as both database and application server at the moment?

I am most familiar with PHP and am okay to use Python.

The physical server is running ubuntu 14.04 and nginx as web server.

The normalized MySQL currently is at 20mb and growing at a rate of 1 mb every month or so.