title: Merge two PDF files into single one using MPDF
body: <p>I am using MPDF library to generate pdf files .I have created two PDF files in my root directory as follows :  </p>
  
  <pre><code>$invoice_nos = ['0'=&gt;'ISE-00000014Y18','1'=&gt;'ISE-00000005Y18'];
  foreach ($invoice_nos as $key =&gt; $invoice_no) {
      $html = 'Invoice No - '.$invoice_no;
      $pdf_file_name = $invoice_no.'.pdf';
      $pdf_file_path = ROOT . '/app/webroot/Service_Invoices/'. DS .$pdf_file_name ;
      ob_start();
      $mpdf = new \mPDF('utf-8', 'A4' ,'','',5,5,36,10,5,4);
      $mpdf-&gt;WriteHTML($html,2);
      ob_clean();
      $mpdf-&gt;Output($pdf_file_name,'f');
  }
  </code></pre>
  
  <p>Now I want to merge these two files into a single file with different pages. How can I do this? I have searched many examples of it but nothing is  working.</p>