An air carrier pilot reported accepting a coded clearance from DEN to PHX while uncertain about the clearance routing because ATC changes them frequently to suit their needs and they are not available to this carrier's crews.

Narrative

We were experiencing very dynamic weather; and heard over the radio that ATC had closed the south gates. We were told to contact Clearance Delivery for a reroute. We did so; and were asked if we had the coded departure routes available. We replied in the affirmative; and were given a clearance that contained the routing of the 'Denver - Phoenix South 1'; which is DENPHXS1 in the FMGC. My First Officer and I looked over the route; but we have no way of knowing what it should be; as it is not published in any of our materials/manuals. I do recall a memo at some time in the past. We talked about asking for a full route; but this is the very reason why the coded routes supposedly exist; and the freq was VERY busy. Anyway; we took the clearance and the routing coincided with what ATC vectored us for. Just outside of GUP; since we usually go in on the EAGUL 3; not the BUNTR which was part of the coded route; we queried ATC; and they said they showed us on the GUP transition to the EAGUL 3 STAR. We changed the arrival and NEVER had a lateral issue; as we were thinking ahead of a potential problem that we were anticipating all along and were ready to query ATC when our box might have differed from what ATC was giving us. We asked the Controller if he knew why the coded route might have been different; and he said that Denver changes and fiddles with the routes on a DAILY basis; and that it is not unusual for the crews to think they have a clearance for one route and actually have a clearance for another. On the ground in DEN; I discussed it with my Dispatcher; who reiterated what the Controller said and made the comment that the coded routes were a violation waiting to happen.