As of this writing, Glitch only supports one backend language, NodeJS. Javascript is great, but a lot of people use other backend languages, one of them being PHP.
While a lot of people might argue that it wouldn’t be worth adding PHP support, PHP still has a lot of users. Right now on Glitch, a lot of people try to use unofficial PHP projects like lamp-poc or php-poc, but these projects are very buggy and make people frustrated. While projects like hello-httpd fix some problems, they don’t solve them all.
Maintaining PHP projects and upgrading PHP would be a great thing and would be very beneficial.