关于mPDF字符串输出格式及前端读取转换方法的技术咨询

Let's break down your questions with clear explanations and practical examples:

1. What format is the string generated by $mpdf->Output('', "S")? #

First, a quick correction: it’s not base68 (that’s likely a typo for base64). When you use the "S" parameter, mPDF returns the raw binary PDF data as a string—this is the exact byte-for-byte content of a PDF file, with no additional encoding applied. It’s the same data you’d find if you opened a PDF file in a hex editor.

2. How to read this string on the backend (PHP)? #

Reading the string is straightforward—just assign the output to a variable, then you can manipulate it as needed:

// Initialize mPDF and build your content
$mpdf = new \Mpdf\Mpdf();
$mpdf->WriteHTML('<h1>Sample PDF Content</h1><p>This is a test document.</p>');

// Capture the raw PDF binary string
$pdfBinaryString = $mpdf->Output('', "S");

// Example: Save it to a local file
file_put_contents('my-document.pdf', $pdfBinaryString);

3. How to handle this string in the frontend for viewing or download? #

Directly passing raw binary strings to the frontend can cause encoding inconsistencies. Here are two reliable approaches:

Option A: Encode to base64 on the backend (simpler for frontend integration) #

First, convert the raw binary string to base64 in PHP—this makes it safe to transmit via APIs or template variables:

$pdfBase64 = base64_encode($pdfBinaryString);
// Pass this $pdfBase64 to your frontend (e.g., via API response or template)

Then use it in the frontend to view or download:

<!-- Embed PDF in an iframe -->
<iframe src="data:application/pdf;base64,<?php echo $pdfBase64; ?>" width="100%" height="600px"></iframe>

<!-- Create a direct download link -->
<a href="data:application/pdf;base64,<?php echo $pdfBase64; ?>" download="my-document.pdf">Download PDF</a>

If using JavaScript (e.g., fetching from an API):

fetch('/api/get-pdf')
  .then(res => res.json())
  .then(data => {
    // Add a viewable iframe
    const iframe = document.createElement('iframe');
    iframe.src = `data:application/pdf;base64,${data.pdfBase64}`;
    iframe.style.width = '100%';
    iframe.style.height = '600px';
    document.body.appendChild(iframe);

    // Add a download button
    const downloadBtn = document.createElement('a');
    downloadBtn.href = `data:application/pdf;base64,${data.pdfBase64}`;
    downloadBtn.download = 'my-document.pdf';
    downloadBtn.textContent = 'Download PDF';
    document.body.appendChild(downloadBtn);
  });

Option B: Send raw binary data and handle as a Blob in frontend #

If you prefer to skip base64 encoding, send the raw binary data with proper headers from PHP:

header('Content-Type: application/pdf');
header('Content-Disposition: inline; filename="my-document.pdf"');
echo $pdfBinaryString;
exit;

Then in frontend JavaScript, convert the response to a Blob for viewing/download:

fetch('/api/get-pdf-raw', { headers: { 'Accept': 'application/pdf' } })
  .then(res => res.blob())
  .then(blob => {
    const pdfUrl = URL.createObjectURL(blob);
    
    // Embed in iframe
    const iframe = document.createElement('iframe');
    iframe.src = pdfUrl;
    iframe.style.width = '100%';
    iframe.style.height = '600px';
    document.body.appendChild(iframe);

    // Create download link
    const downloadBtn = document.createElement('a');
    downloadBtn.href = pdfUrl;
    downloadBtn.download = 'my-document.pdf';
    downloadBtn.textContent = 'Download PDF';
    document.body.appendChild(downloadBtn);

    // Clean up memory after use
    iframe.onload = () => URL.revokeObjectURL(pdfUrl);
    downloadBtn.onclick = () => URL.revokeObjectURL(pdfUrl);
  });

内容的提问来源于stack exchange，提问作者Salar Houshvand