For those of you who, like me, want to put a QRCode on your invoices, you should read on.
Please know that I have no experience in program development…

I first looked around a bit to see what is already available within IP/Mpdf and found the library Barcode in there. In the file “example37_barcodes.php” I found the “QrCode” reference.
I then manually tweaked the dev version of the library QrCode a bit to get this working.

Then I added the “style items - .barCode and/or .barcodecell” in the css file and finally I inserted the following code in my pdf template.

The result:

Note that this is the SEPA/EPC QRCode standard that I have implemented in this example.
See: EPC QR Code - Wikipedia