如何修复MPDF中‘数据已输出，无法生成PDF文件’报错

Problem Statement #

I'm attempting to generate a PDF with the mPDF library using this code:

try { 
    $mpdf = new \Mpdf\Mpdf(); 
    $mpdf->WriteHTML('Hello World'); 
    // Additional code here
    $mpdf->Output("1.pdf", 'D'); 
} catch (\Mpdf\MpdfException $e) { 
    // Note: Using fully qualified exception names is safer
    // Handle exception, log, print, etc.
    echo $e->getMessage(); 
}

Upon execution, I encounter the error: 'Data has already been sent to output, unable to output PDF file'. I’ve already tried ob_end_clean() without success, and tested all solutions from the related question "TCPDF & mPDF error: Some data has already been output to browser, can't send PDF file"—none resolved the issue.

Targeted Solutions to Try #

Since standard output buffer fixes didn’t work, let’s dive into less obvious causes and fixes:

• Hunt for hidden whitespace or characters in included files
  This is one of the most overlooked culprits. Check every file your script includes/requires (configs, helper files, classes) for:

  • Spaces, newlines, or tabs before the opening <?php tag
  • Content after the closing ?> tag (it’s best practice to omit the closing tag entirely to avoid this)
  • Invisible Unicode characters (like zero-width spaces) that might sneak into files via text editors. Even a single unintended character will trigger this error.

• Initiate output buffering at the absolute top of your script
  Instead of only cleaning the buffer, start buffering before any other code runs—make sure there’s nothing (not even whitespace) before this line:

  <?php
  ob_start(); // Start buffering immediately
  
  // Your existing mPDF code here...
  
  ob_end_clean(); // Clean buffer right before outputting the PDF
  $mpdf->Output("1.pdf", 'D');
  

• Suppress PHP errors/warnings temporarily to test
  Even a minor notice or warning (e.g., undefined variable) will send output to the browser before mPDF can generate the PDF. Test by disabling error display temporarily:

  error_reporting(0);
  ini_set('display_errors', 0);
  

  If the PDF works after this, you’ll need to fix those underlying errors (not just suppress them) for a permanent solution.

• Check server-side modules or configurations
  Some server setups, debug tools, caching extensions, or .htaccess rules might inject headers or content before your script executes. Try testing in a clean local environment (like XAMPP/WAMP) to rule out server-level issues. If it works locally, you’ll know to investigate your production server’s configuration.

• Use mPDF's integrated buffer handling
  Wrap your content generation in mPDF-compatible buffer management:

  try { 
      // Start buffer before generating content
      ob_start();
      echo 'Hello World'; // Or your dynamic HTML content
      $htmlContent = ob_get_clean();
  
      $mpdf = new \Mpdf\Mpdf(); 
      $mpdf->WriteHTML($htmlContent); 
      $mpdf->Output("1.pdf", 'D'); 
  } catch (\Mpdf\MpdfException $e) { 
      echo $e->getMessage(); 
  }
  

内容的提问来源于stack exchange，提问作者LeadStar