Persistence of Offline Data Storage

[Accel]

I am currently developing Web Based POS Project for my company, I'm a bit confused on how could i make an Offline Data when Internet Outage /Electricity Outage which is happens quite often in my country.

I am using PHP and MariaDB for developing it and use Codeigniter for framework.

I am contemplating of using JSON or building An Addon for saving Data or even using SQLite ( i don't know if this is even possible as SQLite uses some command MySQL doesn't understand )

any light on this? it will be much appreciated

Thank You

Accel