I just made a pretty interesting discovery in that PHP sessions do not set in IE when there is one or more underscores (_) in the URL.
IE was the last browser I checked an almost completed project in, having developed in Chrome and Firefox. My heart sank when I found I was unable to log in. I hadn’t experienced this issue on any other browser and didn’t have a clue where to even start.
I was able to log in on the staging environment (where I show progress to the client) but not on my local dev so it had to be an environment issue. The only difference between the environments was the URL. I had a play around and found that removing the underscores from my local dev environment URL fixed this issue and I was able to log in. Boom.