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I'm on Fuel 1.4.13 and PHP 7.1.33 and want to upgrade to the latest fuel and PHP 8.1 what are the steps in order so that this is a relatively painless upgrade?
Copy over the ./fuel/codeigniter and ./fuel/modules/fuel folders from the v1.5.2 package. Also copy over the ./fuel/application/config/purifier.php file.
Compare your ./application/config/database.php and ./application/config/config.php files with the v1.5.2 versions - there are subtle differences.
Upgrade your PHP and remember to activate the necessary modules (rewrite etc.)