To sign an existing approval signature field in a PDF Document:

$doc = new PDFDoc($docpath);

// Retrieve the unsigned approval signature field.
$found_approval_field = $doc->GetField($approval_field_name);
$approval_digsig_field = new DigitalSignatureField($found_approval_field);

// (OPTIONAL) Add more information to the signature dictionary.
$approval_digsig_field->SetLocation('Vancouver, BC');
$approval_digsig_field->SetReason('Document approval.');
$approval_digsig_field->SetContactInfo('www.pdftron.com');

// (OPTIONAL) Add an appearance to the signature field.
$img = Image::Create($doc->GetSDFDoc(), $appearance_img_path);
$found_approval_signature_widget = new SignatureWidget($found_approval_field->GetSDFObj());
$found_approval_signature_widget->CreateSignatureAppearance($img);

// Prepare the signature and signature handler for signing.
$approval_digsig_field->SignOnNextSave($private_key_file_path, $keyfile_password);

// The actual approval signing will be done during the following incremental save operation.
$doc->Save($outpath, SDFDoc::e_incremental);

Digitally sign PDF files
Full code sample which demonstrates using the digital signature API to digitally sign and/or certify PDF documents.

The Apryse SDK enables approval signatures in PDF documents using a Digital Certificate, in accordance with the latest PDF specification. By leveraging public key infrastructure (PKI) technology, with a certificate issued by a trusted certificate authority (CA), a signer can use a certificate-based digital ID to guarantee the authenticity of a signature. Placement of a digital signature using a certificate can also guarantee that a document was not modified since the signature was placed, ensuring the authenticity of the document.

Above is an example of a document containing a certified signature, guaranteed by a certificate generated by Apryse.com.