Are you looking for something like this?
set @rank:= 0;
SELECT (@rank:= @rank+ 1) AS Rank , users.* FROM users ORDER BY reputation DESC;
This will increment @rank for each resulting record. For a specific user use:
SELECT T.reprank
FROM (SELECT @rank := @rank + 1 AS reprank,
users.*
FROM users,
(SELECT @rank := 0) rnk
ORDER BY reputation DESC) AS T
WHERE id = 23