I'm probably doing something stupid but it seems to be autoloading my helper class

That's seem to be simple indeed,
every time you intent to extends a core class, this one override automaticaly without any special include. Just use the class prefix defined on config.php file and it's done.

I think you sould create a custom library instead and drop that file on "library/" folder, so it would be easy for you to include it on each controllers or on specifics.

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