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You should start by upgrading to 2.2.6: https://codeigniter.com/userguide2/insta...ading.html

Once you've done that, you should go through the process to upgrade to 3.0.4: https://codeigniter.com/user_guide/insta...ading.html

Most of the errors you've posted so far should be fixed by the combination of changing the driver to mysqli and upgrading to 2.2.6. In the long term, though, 2.x is no longer supported, so you would be better off continuing the upgrade path to 3.0.4. The main reason you should start by upgrading to 2.2.6, though, is that you may have an easier time getting your application to work in 2.2.6 than in 3.0.4, so you could at least get a large number of security fixes in place and make some progress.

You can probably go straight from 2.1.3 to 2.2.6 following these instructions: https://codeigniter.com/userguide2/insta...e_226.html

Note that the step to upgrade from 2.2.x to 3.0.0 is especially important when moving your application to 3.0.4: https://codeigniter.com/user_guide/insta...e_300.html

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