CODE EXAMPLE FOR PHP

php check if headers already sent

<?php
  
//You can use 'headers_sent' function 

// If no headers are sent, send one
if (!headers_sent()) {
    header('Location: http://www.example.com/');
    exit;
}

// An example using the optional file and line parameters
// Note that $filename and $linenum are passed in for later use.
// Do not assign them values beforehand.
if (!headers_sent($filename, $linenum)) {
    header('Location: http://www.example.com/');
    exit;

// You would most likely trigger an error here.
} else {

    echo "Headers already sent in $filename on line $linenum
" .
          "Cannot redirect, for now please click this <a " .
          "href="http://www.example.com">link</a> instead
";
    exit;
}

?>