PHP - cannot modify header information - headers already sent by (output started at

I am working with PHP and I got this message

cannot modify header information - headers already sent by (output
started at

after executing a code which looks like this:

    if (condition) {

        if (condition) {
            //Statement
        }


        $to = $_POST['email'];
        $subject = "Registration Confirmation";

        $message = '
                
                    
                        
                    
                
            ';


        // Always set content-type when sending HTML email
        $headers = "MIME-Version: 1.0" . "\r\n";
        $headers .= "Content-type:text/html;charset=UTF-8" . "\r\n";

        // More headers
        $headers .= 'From: ' . "\r\n";

        mail($to,$subject,$message,$headers);


        header('Location: reset.php?username='.$username);
        exit();

    }else {
        //statement
    }

Based on this answer from Stackoverflow, I think the error is coming from the HTML code in the variable $message. I don't really know how I can modify the content of that variable to avoid the error.

Kindly help me solve this problem.

Answer

From php documentation:

Remember that header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP.

So move header('Location: reset.php?username='.$username); before any other kind of output.

By the way, this question was already answered: