Cannot modify header information

I was trying to upload a header and it would not let me crop it. So I did some research and applied a code to tell me whats wrong and this is what I got...

//allow redirection, even if my theme starts to send output to the browser add_action('init', 'do_output_buffer'); function do_output_buffer() { ob_start(); }
Warning: Cannot modify header information - headers already sent by (output started at /home/ieshoppers/public_html/iheartpawspetrescue.com/wp-content/themes/my-lovely-theme/functions.php:21) in /home/ieshoppers/public_html/iheartpawspetrescue.com/wp-includes/pluggable.php on line 1121

As I can see in your error, you are talking about a WordPress website. What does it have to do with Open Cart?

Andy