The default file editor/view in the preferences is used when you attempt to open a file that doesn't already have a windows file association, it would default to the internal editor but with a the default editor defined it is used.
I suspect you have .php files set to open in notepad within windows.
To change which program is used just for FlashFXP you need to add custom file associations within FlashFXP.
From the main menu
Options >
File Associations
Then on File Associations dialog
- Click Add a new entry
- Define your file pattern masks, i.e. "*.php"
- Set Open with to "Internal Editor" (since your default editor/viewer is set to Notepad++ it will be used instead.)
- Check the use to edit box
- Click Save
