Qualified electronic signatures in PDFs using PHP and a USB token/smartcard

Adding a digital signature to a PDF file is relatively easy using a library such as TCPDF, as long as your personal certificate is stored somewhere on your server/computer. However, in order for a digital signature to be considered “Qualified” in the European Union (which -technical details aside- means it holds the same value as a handwritten signature) the certificate needs to be stored in a Secure Signature Creation Device, such as a smartcard or a USB token.