Cannot modify header information - headers already sent by... WordPress Issue

If you are trying to redirect to another page from your current page, where you have impose a condition or without condition, then use this code. E.gs you have two pages A.php, & B.php and currenty you are in A.php where you want to go to other page B.php on clicking the BUTTON.

   if(isset($_POST['save_btn']))
    {
        //write some of your code here, if necessary
        echo'<script> window.location="B.php"; </script> ';
     }

Your theme is printing output (text) to the browser, but then for some reason WordPress is redirecting the user (with wp_redirect) away from that page before the whole page is rendered. You can't start printing output and then redirect, or you'll get the error you see. That's what Paul Grime was getting at in his comment.

Ken White commented with a reference to a post with a similar problem. I've fixed this in my own experience by buffering the output of the script.

In your theme's functions.php file (which gets included every time your theme's pages load), put the following:

//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();
}

Now, even if part of your theme starts to send input to the browser, PHP won't send that text until the page is fully loaded, which allows WordPress to redirect users, if necessary, as part of its own logic.