How to digital sign a PDF with PHP [closed]

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After so开发者_JAVA技巧me searching how I could digital sign a PDF document with PHP I couldn't really find a working solution. Does anybody know how to do this? Or do you know a third-party service which can sign PDF's for us?

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Setasign appears to offer a commercial solution for signing PDFs already created - http://www.setasign.de/products/pdf-php-solutions/setapdf-signer/sign-pdf.php

Alternatively, TCPDF (Google it, it won't let me post a second URL), allows you to generate signed PDFs on the fly.