Hi fcarew,
Thank you for using Mail-in-a-Box. Please keep in mind, that the box is designed to be a “just-works” solution for non-advanced users. Implementing things as PHP or something would make it more complicated to reproduce errors by the community. Thats why we are not integrating much options for modifying 
If you want to modify your box by activating PHP on your domain, you are able to do this. But keep in mind - if you do this - you will be on your own. If anything crashes or does not work as excepted, the community and neither @JoshData may not be able to help you.
A User has posted how to do this: Is atleast being able to toggle PHP planned?
But keep in mind: It’s unsupported 
And please … there are so many theards in this forum which users ask the same questions … and they are already answerd by @JoshData or the community. And I think @JoshData hasn’t changed his opinion in this short time. 