DOMPDF: Unable to stream pdf: headers already sent

Question

This question has been raised hundred of times in various fora; some have been solved, some not. I checked plenty of resources, but my issue could not be resolved. I am generating php page using smarty template based on html form input and uploaded files. Using dompdf, I want to save the generated page as a pdf file. When the user submits the multipart/form-data, data is posted to itself. Then it undergoes validation process. When all is fine, a new page is generated using a template file. There is no output, instead, dompdf utilizes the template file to stream the pdf file. After solving several stages of problems such as "DOMPDF not found", insufficient memory etc, I am now stuck with "Unable to stream pdf: headers already sent" error. One of the most common problems is presence of line break, white space or any output being before stream() is called. I checked for white space before and after ?. There are nor print_f or echo statements either. How can I troubleshoot this problem? Where does the problem lie...in the smarty template file or the php file itself? Here is the code:

require_once("dompdf/dompdf_config.inc.php");
spl_autoload_register('DOMPDF_autoload');
$html = $smarty->fetch('index.tpl');
$dompdf = new DOMPDF();
$dompdf->load_html($html);
$dompdf->set_paper('a4', 'portrait');
$dompdf->render();
$dompdf->stream("newfile.pdf");

Answer 1

simple solution :

write below lines before stream, it will show where exactly new line or space is coming

$f;
$l;
if(headers_sent($f,$l))
{
    echo $f,'<br/>',$l,'<br/>';
    die('now detect line');
}