Having seen the hairballs of creative PHP in the wild, I'd think the only sane way to do this would be some sort of deprecation warning whenever a symbol lookup matched only case insensitively.
Like the ones that they introduced to fix array[key_without_quotes] where key_without_quotes was mapped to a string if it was not a defined constant and a NOTICE was issued? The first thing everyone did was turn off E_NOTICE since practically all code emitted that notice. It took years until you could run apps with E_NOTICE turned on :)
True: I spent years swimming upstream trying to get PHP developers to log sensibly and actually fix problems rather than suppressing the notices, which is probably why I'm much happier with the Python community.