Unable to stream pdf: headers already sent when using wordpress

Question

I'm trying to use dompdf on my plugin to create a save as pdf functionality,but I'm still getting Unable to stream pdf: headers already sent.

I've done some research and the solutions won't work. I've also use exit() as one of the answers suggest but it doesn't work.

Here is my current code

include(WPtest_PLUGIN_PATH.'/admin/includes/dompdf/lib/html5lib/Parser.php');
include(WPtest_PLUGIN_PATH.'/admin/includes/dompdf/lib/php-font-lib/src/FontLib/Autoloader.php');
include(WPtest_PLUGIN_PATH.'/admin/includes/dompdf/lib/php-svg-lib/src/autoload.php');
include(WPtest_PLUGIN_PATH.'/admin/includes/dompdf/src/Autoloader.php');
Dompdf\Autoloader::register();

use Dompdf\Dompdf;

class WPtest_Save_PDF{

    //use Dompdf\Dompdf;
    function __construct(){
        add_shortcode( 'save_me',array($this,'print_callback'));
    }       


    function print_callback(){

        if(isset($_GET['print']))
        {


            $dompdf = new Dompdf();

            $dompdf->loadHtml("test");

            // (Optional) Setup the paper size and orientation
            $dompdf->setPaper('A4', 'landscape');

            // Render the HTML as PDF
            $dompdf->render();

            // Output the generated PDF to Browser
            $dompdf->stream();

            exit();
        }

    }
}

new WPtest_Save_PDF;

Answer 1

your wordpress already sent some data to the browser, and when any data is sent, the headers are sent as well.

I would suggest you to use buffers (ob_start, ob_end_flush, ...) but it's on a general purpose, I do not know if it is appliable in your case, i'm not really working with wordpress structure.