It only returns the error when I try to access php files. For example, localhost:8000/favicon.ico works. If I create an html file at localhost:8000/test.html that also works. But any php files I create return the 503 error.
This shows up in the logs whenever I try to access a PHP file:
[Tue May 28 13:24:02.426930 2019] [proxy:error] [pid 10330:tid 139800397223680] (111)Connection refused: AH00957: FCGI: attempt to connect to 127.0.0.1:9000 (127.0.0.1) failed
[Tue May 28 13:24:02.426967 2019] [proxy_fcgi:error] [pid 10330:tid 139800397223680] [client 172.18.0.1:52424] AH01079: failed to make connection to backend: 127.0.0.1