DITA does not really support the concept of variables, other than conref attributes. Apparently the truly DITA-proper thing to do is to use conref attributes at the <ph> level.
§15.11.1 Understanding how Mif2Go represents variables in DITA
§15.11.2 Specifying how to treat FrameMaker variables
§15.11.3 Treating FrameMaker variables as conrefs
§15.11.4 Retaining format properties of user variables in DITA