Read and substitute constants in XML during XSL transformation

I have an xml with the structure,

<root>

  <constant name="user">johndoe</constant>
  <constant name="server">1</constant>

  <connection>
    <userName>${user}</userName>
    <port>1234</port>
    <server>matrix.${server}.abc.com</server>
  </connection>

</root>

I'm extracting the information using XSLT to a CSV structure. How do I replace the constant names with their actual values? If this can be done in XSL, then I also have a few cases of 'nested' constants like,

<constant name="a">123</constant>
<constant name="b">10${a}</constant>