There's two different types of production codes, and most of people on these wikis don't seem to know that. One type isn't even technically production codes *most, not all
This is much more fairly common with the 11-minute cartoons than 22-minute ones like Gravity Falls or Kim Possible, because there's a production code that you guys seem to find much easier that combines two 11-minutes into a half-hour. This is a more common version and is often used more than the real production codes.
What are the real production codes? 11-minute shows are produced separately, not together. I don't know the actual order thanks to the wiki not using them (either that or there being not enough "proof" for them since they are usually only found via storyboards or other production art --- often they are produced in a much different order than they are released). Thus, I'll pretend the a-b production order is the real order just for this example.
Rollercoaster would be 101, Candace Loses Her Head would be 102, and half-hours also cover a single production code too. (I believe Flop Starz was the actual 101? Not sure) BUT! BUT BUT BUT --- I'm unsure what it is for Phineas and Ferb in particular, but the shows over at DTA have a little prefix to make the production code apply to this specific show. It's three numbers and a letter. Wander Over Yonder's is 345W-, Gravity Falls' is 618G-, and it is continued with the production number. (update, source at bottom, Phineas and Ferb's prefix is 631D)
This originally fooled me, because I recall The O.W.C.A. Files' production code (or at least one part of it, since the two parts of an hour-long special are in fact produced separately) was like, something over 460, and I thought there'd be that many half-hours in Season 4. But that's actually not true.
Another fun fact to this is that 100 is the theme song for the DTA shows. Since the intro partially changes every season, I'm not sure if it's 200 and 300 and 400 in those cases, or what it'd be for any special intros (like the Halloween ones) but I know 100 applies to the main one shown at the beginning of the show at least.
Just thought I'd let you guys know since when I added The O.W.C.A. Files' production code, it got removed and replaced with 43x/43x, and you guys are still using the half-hour codes instead of the production codes.
Obviously, it'd be way too much work to add (let alone find) all the available production codes to the articles, so go ahead and just leave it if you want. Also thought this'd be worth a discussion.
EDIT: Found the right image. Production code prefix is 631D.