Codeigniter 2.2x IS COMPATIBLE with PHP7.1+.
You only need to remember:
- Models/classes and so on are case-sensitive and have to start from capital letter
- The same with names of files in application folder
- There are problems with mysql (very old, not used now driver) so you have to use mysqli driver (but it is already implemented in CI2, check if you're not using it already)
- if you have session table sometimes you'll have to truncate this table, clear cache and populate it again.
- Check compatibility of software in third_party folder if you have problems - CI is not responsible for that issues.
In my case everything is working smoothly with the newest version of bootstrap & jQuery installed. However, I'm not using hooks, advanced functions and so on.
NOTE, that in most cases you can force server to use older php version using .htaccess and AddHandler command:
AddHandler application/x-httpd-php56 .php
This is last version of PHP before PHP7.
And last but not least - of course I don't recommend to use this very old version of CI, however in my opinions - in my cases - there is no sense to migrate from CI2x to CI3, in 2018 it is better to wait for CI4 and then migrate.