Settings/Preferences | Editor | File Types | Ignore files and folders Settings/Preferences | Editor | File Types | Ignore files and folders at the bottom - add the *-backup.php template (or any other template you need).
PLEASE NOTE: this affects all projects, as this is an IDE parameter.

Alternatively (have not tried myself, but should work):
Settings/Preferences | Editor | File Types | Recognized File Types | Text
Add the *-backup.php template.
This will also affect all projects .. but instead of throwing it, it will process them as text files, so there is no indexing / code completion / syntax highlighting / etc, still having such files in the Project View tree (so you can edit / delete / upload / etc, if necessary).
LazyOne Jun 18 '13 at 12:23 2013-06-18 12:23
source share