I got this error suddenly when i login and register....
it is problem MyBB or bug or something is wrong....
Reupload ./inc/functions.php from a fresh download.
I dont understant???
I no see /inc/functions.php in my localhost...
Note the dot before the forward slash.
Replace the file /path/to/mybb/inc/functions.php with a fresh file from here:
http://www.mybb.com/downloads