How can one use PHP to extract digital signature details from a PDF document?

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One can use the following steps to extract digital signature details from a PDF document using PHP:

1. Install a PDF library in PHP, such as TCPDF or FPDI.

2. Load the PDF document using the library, and get the signature field objects from the document.

3. Iterate through the signature field objects and extract the digital signature details, such as the signer's name, signing date, and signature validity status.

4. Use the library functions to verify the signature, and display the results to the user.

Here's an example using TCPDF library:

require_once('tcpdf/tcpdf.php');
// Load the PDF document
$pdf = new TCPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);
$pdf->setSourceFile('document.pdf');

// Get the signature fields from the document
$signatureFields = $pdf->getSignatureFields();

foreach ($signatureFields as $signatureField) {
  // Get the signature object from the field
  $signature = $signatureField->getSignatureObject();

  // Extract the signature details
  $signedBy = $signature->getName();
  $signedOn = $signature->getDate();
  $validity = $signature->isValid() ? 'valid' : 'invalid';

  // Display the signature details
  echo 'Signed by: ' . $signedBy . '<br>';
  echo 'Signed on: ' . $signedOn . '<br>';
  echo 'Validity: ' . $validity . '<br>';
}

Note that the exact code may vary depending on the PDF library used and the specific signature details to extract.