Unable to stream pdf: headers already sent

// Include autoloader
require_once 'dompdf/autoload.inc.php';
require_once 'dompdf/lib/php-font-lib/src/FontLib/Autoloader.php';
require_once 'dompdf/lib/php-svg-lib/src/autoload.php';
require_once 'dompdf/lib/html5lib/Parser.php';


$folder='http://11111111/pdfs';
$pdfid=$resultpackages['package_id'];


// Reference the Dompdf namespace
use Dompdf\Dompdf;
use Dompdf\Options;

$options = new Options();
$options->set('defaultFont', 'Courier');
$dompdf = new Dompdf($options);

// Instantiate and use the dompdf class
$dompdf = new Dompdf();

// Load content from html file
$html = file_get_contents("http://********.com/pdf.php");
$dompdf->loadHtml($html);

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

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

// Output the generated PDF (1 = download and 0 = preview)
$dompdf->stream("package",array("Attachment"=>0));