สอบถามเรื่อง PHP มีวิธี add digital signature อย่างไรครับ

/*
NOTES:
 - To create self-signed signature: openssl req -x509 -nodes -days 365000 -newkey rsa:1024 -keyout tcpdf.crt -out tcpdf.crt
 - To export crt to p12: openssl pkcs12 -export -in tcpdf.crt -out tcpdf.p12
 - To convert pfx certificate to pem: openssl pkcs12 -in tcpdf.pfx -out tcpdf.crt -nodes
*/

// set certificate file
$certificate = 'file://data/cert/tcpdf.crt';

// set additional information
$info = array(
    'Name' => 'TCPDF',
    'Location' => 'Office',
    'Reason' => 'Testing TCPDF',
    'ContactInfo' => 'http://www.tcpdf.org',
    );

// set document signature
$pdf->setSignature($certificate, $certificate, 'tcpdfdemo', '', 2, $info);

// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
// *** set signature appearance ***

// create content for signature (image and/or text)
$pdf->Image('images/tcpdf_signature.png', 180, 60, 15, 15, 'PNG');

// define active area for signature appearance
$pdf->setSignatureAppearance(180, 60, 15, 15);

// *** set an empty signature appearance ***
$pdf->addEmptySignatureAppearance(180, 80, 15, 15);

openssl pkcs12 -in tcpdf.pfx -out tcpdf.crt -nodes
// openssl pkcs12 -in ไฟล์ pfx -out ไฟล์ cert -nodes

// set certificate file
$certificate = "file://{$_SERVER['DOCUMENT_ROOT']}/path/mycert.crt";