PHP Tic-Tac-Toe MinMax algorithm implementation

I got an assignment creating a Tic-Tac-Toe game base on PHP. The task required that the player won't be able to win the game, meaning either the computer wins or it ends in a draw.

I wrote a Tic-Tac-Toe game.

I got an assignment creating a Tic-Tac-Toe game base on PHP. The task required that the player won't be able to win the game, meaning either the computer wins or it ends in a draw.

There was only one clue: "use MinMax algorithm".
Since I wasn't familiar with MinMax algorithm I had to read about it. I found two sources one on Wikipedia, and [another article by Paulo Pinto] (http://www.progtools.org/games/tutorials/ai_contest/minmax_contest.pdf "Introducing the Min-Max Algorithm by Paulo Pinto").

I enjoyed this task and once I finished it, I got a feedback how to make it better. My code is available on GitHub