I don't know what happened but syntax highlighting for one php file stopped to working and also icon next to the file has changed. It shows it's text file instead of PHP .
How can I fix that ?
The easiest way to fix this is to go to File > Settings on Windows (or PHPStorm > Preferences on MacOS), then search for "FILE TYPES".
Scroll down to the troublesome filetype (in this case PHP).
Then look for the expected file extension (in this case .php).
If you see it's missing (as above), then just add it by pressing the + button under "Registered Patterns". Then in the "Add Wildcard" window that opens you would type: *.php
PHPStorm will then tell you that it's mapped to another filetype (the one that's caused this problem), and would you like to move it to this one instead. Simply click "Yes" and it will fix everything. You're all done!
This is much easier than hunting around the other file types for broken one to delete.