Message: Cannot modify header information - headers already sent by

This simply means that something was sent to the browser prior to the headers being sent as part of your request. The two times you'll see this the most are:

1 - You've had another error that is displaying something on your page. Fix the error and it will go away.
2 - You have a space or a blank line or other text before the opening `<?php` of a file. Go through your controller, libraries, models, etc, that are being used on that page and make sure there isn't anything before that tag at the top of the file.

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