Upon trying to run some migrations in FuelPHP via command line with Oil I ran into this error message containing the error
Uncaught exception Fuel\Core\Database_Exception: No MySQLi Connection
I discovered that when running from the command line my normal php.ini was not being run. PHP was using the defaults, including the default mysql socket which was in the wrong place.
In my case I created a symlink to redirect which seems to have done the trick. I am running Mamp on OS X in default locations so your directories may differ.
sudo ln -s /Applications/MAMP/tmp/mysql/mysql.sock mysql.sock
There may be a better fix and this may be a symptom of something else being ‘off’. I initially created a /etc/php.ini with the proper socket location directive but this did not work out properly, possibly due to permissions or something.