How to show the full file path in PHPStorm?
The top bar should show you where your file is located:
View > Appearance > check 'Navigation Bar'
Unfortunatly the only option you have is for it to show the path for non-unique filenames.
Solution
There is now an option to do this in the editor settings labeled "Always show full path in window header"
Settings/Preferences | Appearance and Behavior | Appearance | UI Options
Documentation
Always show full path in window header
Show the full path to the project and to the current file in the main window header. When disabled, the header will show only the name of the project and file.
Reference: https://www.jetbrains.com/help/idea/settings-appearance.html#ui
Note: Another useful setting for those looking for visibility of file paths is "Show full path on mouse hover". This is found under ( Settings/Preferences | Editor | General | Editor Tabs )