I had one problem while calling controller from another controller,one of the user of stackoverflow suggested to use require_once but another person said it's not a good idea now i'm confused, which is safe ? Using require_once or write a function in helper?
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I would say in general that calling a controller from within another controller is not a best practice. If two controllers need to use the same logic then maybe that logic should be in a separate place. This can be done by creating your own library and loading it within both controllers.
Check out the link below to learn more about creating libraries in CodeIgniter.