What are the methods to pass a variable through redirect() CI?

Question

CodeIgniter

I'm doing a project on codeIgniter 3.1.11
There is a classic task to generate a confirmation letter from the url to the mail.
After the form inserts the fields in the database, I redirect to the confirmation page and send an email with the code. The question is how to pass a variable to the code to compare it with the code that the user received? Like flashcookies. But the question arises: is it safe to use cookies for such variables?