How to replace photo in moderncv classic with a QR code?

\photo command wants a file suitable to be included with an \includegraphics command. Therefore, I think the easiest solution would be to create your own qrcode within an standalone document and use the result as photo in moderncv.

%File qrcode.tex -> qrcode.pdf
\documentclass{standalone}
\usepackage{qrcode}
\begin{document}
\qrcode[hyperlink,height=64pt]{http://www.ctan.org}
\end{document}

\documentclass[11pt,a4paper,sans]{moderncv}
\moderncvstyle{classic}
\usepackage{qrcode}
\usepackage{graphicx}

\firstname{John}
\familyname{Doe}

\address{83 Fancy Avenue}{Noweheresville}{Gotham City 24061} 
\phone[mobile]{123 456 7890}
\email{[email protected]}

\photo[64pt][0.5pt]{qrcode}

\begin{document}
   \makecvtitle
\end{document}

Update: All in one ;-)

With filecontents package and an \immediate command it's possible to join both files into one and compile both together:

\documentclass[11pt,a4paper,sans]{moderncv}
\moderncvstyle{classic}
\usepackage{qrcode}
\usepackage{filecontents}

\begin{filecontents*}{myqrcode.tex} %<- This is the qrcode file name
\documentclass{standalone}
\usepackage{qrcode}
\begin{document}
\qrcode[hyperlink,height=64pt]{http://www.ctan.org}
\end{document}
\end{filecontents*}

\immediate\write18{pdflatex myqrcode.tex}

\firstname{John}
\familyname{Doe}

\address{83 Fancy Avenue}{Noweheresville}{Gotham City 24061} 
\phone[mobile]{123 456 7890}
\email{[email protected]}

\photo[64pt][0.5pt]{myqrcode} %<- Insert previous name

\begin{document}
   \makecvtitle
\end{document}

In the code of class moderncv, style classic you can find an if-then-else construct checking if a \photo is defined (printing it) or not (doing nothing). I added inside the empty case a new if-then-else testing, if a new command \qrphoto is defined. If it is defined the code prints it, in the other case it does nothing.

The new command for the \qrphoto is:

\NewDocumentCommand{\qrphoto}{O{64pt}O{0.4pt}m}{\def\@qrphotowidth{#1}\def\@qrphotoframewidth{#2}\def\@qrphoto{#3}}

With this command I define the needed values \@qrphotowidth etc. for later usage.

Now we can patch the original command with

\patchcmd{\makecvhead}%
  {%
    \ifthenelse{\isundefined{\@photo}}{}%
  }% code to patch
  {% new code <=========================================================
    \ifthenelse{\isundefined{\@photo}}%
      {%
        \ifthenelse{\isundefined{\@qrphoto}}%
        {}%
        {%
          \if@left%
            \hspace*{\separatorcolumnwidth}\fi%
          \color{black}% <======================== to get a black qrcode
          \setlength{\fboxrule}{\@qrphotoframewidth}%
          \ifdim\@qrphotoframewidth=0pt%
            \setlength{\fboxsep}{0pt}\fi%
          \raisebox{1cm}{\framebox{\qrcode[hyperlink,height=\@qrphotowidth]{\@qrphoto}}} 
        }
      }%
  }% end new code <=====================================================
  {}% success
  {\fail}% failure

The command \raisebox[1cm] is needed to get the qr-code on the right position (I did not search for the reason, why I need it here. The used value is okay for sizes of the resulting qr-code of 2cm (I used for the example code) or more. If your qr-code should be smaller you need to change my used value of 1cm to a lower value. You will have to try it out.

As you can see I deleted the command \includegraphics. With command \qrphoto \qrcode... from package qrcode.

With the command

\qrphoto[2cm][0.5pt]{https://tex.stackexchange.com/questions/474546/}

for example you can add an qr-code-image with a hight and width of 2cm, a frame around it with a line of 0.5pt and the content of the qr-code-image is https://tex.stackexchange.com/questions/474546/ (this question).

So with the following complete MWE

\documentclass[11pt,a4paper,sans]{moderncv}

\moderncvstyle{classic}

\usepackage{graphicx}
\usepackage{qrcode}

\firstname{John}
\familyname{Doe}

\address{83 Fancy Avenue}{Noweheresville}{Gotham City 24061} 
\phone[mobile]{123 456 7890}
\email{[email protected]}

\makeatletter
\NewDocumentCommand{\qrphoto}{O{64pt}O{0.4pt}m}{\def\@qrphotowidth{#1}\def\@qrphotoframewidth{#2}\def\@qrphoto{#3}}
% to patch the code of moderncv, version 2.0.0
%\usepackage{etoolbox} % already loaded in moderncv <===================
\patchcmd{\makecvhead}%
  {%
    \ifthenelse{\isundefined{\@photo}}{}%
  }% code to patch
  {% new code <=========================================================
    \ifthenelse{\isundefined{\@photo}}%
      {%
        \ifthenelse{\isundefined{\@qrphoto}}%
        {}%
        {%
          \if@left%
            \hspace*{\separatorcolumnwidth}\fi%
          \color{black}% <======================== to get a black qrcode
          \setlength{\fboxrule}{\@qrphotoframewidth}%
          \ifdim\@qrphotoframewidth=0pt%
            \setlength{\fboxsep}{0pt}\fi%
          \raisebox{1cm}{\framebox{\qrcode[hyperlink,height=\@qrphotowidth]{\@qrphoto}}} 
        }
      }%
  }% end new code <=====================================================
  {}% success
  {\fail}% failure
\makeatother

%\photo[64pt][0.5pt]{example-image}
\qrphoto[2cm][0.5pt]{https://tex.stackexchange.com/questions/474546/} % 


\begin{document}
  \makecvtitle
\end{document}

you get the following result:

Please note that you can only print a photo or a qr-code. If you have a defined \photo and \qrphoto only the photo is printed!