Basically what you should do is host your PHP file on your own web server, then have your game make a GET request to
https://[your domain name]/iIndex.php?data=UserLeonFinishedScene1
So when someone plays your game on here, their browser downloads all the actual game stuff from Itch, but those game files then tell it to send some data to your server. If you already have the game working on your own website then you should be able to basically just upload the same client files to Itch, although you might have to set up CORS headers on your sever or something.