You immediately find in the docs the way handle to userid:
qa_handle_to_userid($handle)
returns the userid for the user identified by $handle
or null
if no user matches. This requires Q2A 1.6+, but see also qa_handles_to_userids(...)
and qa_userids_to_handles(...)
which were added in Q2A 1.5.
http://www.question2answer.org/functions.php
With v1.6 there was a new function implemented for userid to handle:
function qa_userid_to_handle($userid)
which is not yet in the docs.