It is quite likely the sites that are hosting your games (such as itch.io) will only allow connections to external sites via HTTPS. This means your hosting site (where you have your PHP) must serve the data via the HTTPS protocol. You'll need to apply an SSL certificate to your domain, so you can serve your data over HTTPS. You'll also need to update any code references in your game from HTTP to HTTPS.
How you install the SSL certificate is purely down to which web hosting company you are using - look up SSL in their help system. SSL can be done completely for free, but most hosts will charge you an admin fee (sometimes ongoing), and will often want you to buy the SSL Certificate from them for an inflated price.
Here's an example SSL help page from Dreamhost (I'm not recommending them - just giving an example of a host that offers a free SSL option): Please login to see this link.