@mishavee is correct in that you need some way to tell Glitch what to do when your project starts up. The existing PHP examples largely use a glitch.json file which is an alternative to package.json but for non-Node projects. Additionally you need some sort of server to run your PHP code, since Glitch assumes that Node is your server. Most of the examples I’ve worked with include specific Apache web server information in the project, so starting out by Remixing one of those projects makes sure you’ve got those additional required files - otherwise Glitch doesn’t know what to do.
I’d suggest Remixing one of the projects in https://glitch.com/search?q=php&activeFilter=project to give you a solid starting point.
Hope this helps!