An PHP online editor / IDE ...
It would be great that the online visual editor would color php functions that are deprecated in the php version choosed.
Under "files - code editor" you have already created a basis, an approach. This code editor would only have to be specially designed for php (of course much better with additional languages). With functions like code completion, code folding, debugger, CLI access, ftps etc. Then you could also develope online in Plesk instead using third-party suppliers. Finally you have Plesk server administration, third-party apps and (Plesk) development platform, all together in the own server.