'How can I show output message for user to see after registration

I made registration form,but I don't know how to display message to user if email that he inserted is already in use

REGISTER.PHP

<div class='row'>
            <div class='col-12'>
              <div class='odgovor'></div>
            </div>
</div>

I want to put message of filling form(successful or unsuccessful) in this class ODGOVOR

REGISTRACIJA.PHP -->this page is used for processing form input

php

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