Soo... the application isn't really working. Yes thats not a good start.
In folder src is finished class that is responsible for handeling the logick. In folder tests is a test for this class. That is where the problem begins. I can't remember how to connect 2 classes from different files in vanila PHP. Since this task isn't needed to be 100% finished I hope it's not that much of a problem since we should mainly focus on how the code is written. Since I couldn't get GameOfLife class into Test class i decided to at least prepare some test templates for test that I would like to have there.
But if you want to check if the application works i made a index.php. If you run this file it will guide you how to give it a path to the file and present you the results in JSON.
Yeah I didn't even touch any HTML or CSS... so here is a json formater: https://jsonformatter.curiousconcept.com/