If you have exif.so or php_exif.dll enabled under php.ini dynamic extensions, but you still have problem running any funtion, it is possible that you may need to change order of extensions in php.ini.
In many PHP installation packages the extensions are listed alphabetically. However, exif is dependent on mbstrings to be loaded first. Yet, with alphabetic ordering the exif is listed before mbstrings and it causes the error. Load mbstrings extensions before exif.
Thus, do put exif as the very last extension to load and you should be fine.