title: mPDF error: Some data has already been output to browser, can't send PDF file
body: <p>I am getting such error whn I try to use mPDF. 
  <strong>mPDF error: Some data has already been output to browser, can't send PDF file</strong></p>
  
  <p>Here is the code:</p>
  
  <pre><code>&lt;?php
  include("mpdf60/mpdf.php");
  
  $mpdf=new mPDF('c','A4','','' , 0 , 0 , 0 , 0 , 0 , 0); 
  
  $mpdf-&gt;SetDisplayMode('fullpage');
  
  $mpdf-&gt;list_indent_first_level = 0;  // 1 or 0 - whether to indent the first level of a list
  
  $mpdf-&gt;WriteHTML(file_get_contents('invoice.html'));
  
  $mpdf-&gt;Output();
  
  
  $to      = $_POST["email"];
  $subject = 'the subject';
  $message = 'hello';
  $headers = 'From: narehh@mail.ru' . "\r\n" .
      'Reply-To: narehh@mail.ru' . "\r\n" .
      'X-Mailer: PHP/' . phpversion();
  
  mail($to, $subject, $message, $headers);
  </code></pre>
  
  <p>Have you ever had this problem? How to solve it?
  Thanks</p>