I started using PhpED pro with version 5.x and I'm now using
version 8.1.

A lot of useful features have been added; one more useful feature
would be the ability to set an opened file to "Read-only" by right-clicking
the file's tab and selecting a "Read-only" option, instead of having to navigate
through the shell menu and change the file's attribute here; as well, having
a "read-only" indicator at the bottom of the editor when that file has the focus
would be a quick indicator of that file's read-write status.

This might help prevent unintentionally editing a file opened for viewing but
not editing, while editing a similar adjacent opened file.

It would be nice to have a way of quickly toggling a files readonly status; a common request in this forum and surely easy to implement.

As for knowing when a file is readonly, PhpED does display R/O in the bottom right-hand corner for readonly files plus the PhpED editor does not allow readonly files to be changed.