This was my first dive into writing a PHP plugin for WordPress. While a co-worker wrote most of it and should receive the credit, I learned and did much tweaking/building new features. The purpose of this experience was to manage time off requests on the company intranet (named "Aisle3").
There are two portions: the user page (hey, I need to request some time off; also it'd be nice if I already knew how much time I had used) and the supervisor page (acts as a dashboard for all requests on that supervisor's team).
There is a third level - the "Operations" view which can see/manage all requests across the company. All of these functions and many more (including the ability to export the requests to an Excel sheet) were built into these two pages. I was responsible for the UX (general layout) and many of the design decisions. Also, I wrote all necessary JS and, like I earlier mentioned, edited/wrote a lot of PHP. It was a great project.