So, this came up in game. An enemy cast wall of fire as a ring, with the heat side being the inside. The question that was brought up was, “do you take damage when leaving the fire wall ring?” I found this on RPG.stackexchange:

“you cast the spell as a ring with the damage side in, and everyone standing exactly where the wall forms takes 5d8 damage or half on a Dex save; meanwhile anyone surrounded by the ring takes no damage immediately, but will start taking 5d8 damage (no save) at the end of their turn, every turn, until the spell ends or they exit through the wall (taking another 5d8 fire damage, no save, for the effort).”

However, the argument was made that the spell only says you take damage when entering the wall, not exiting. To me, it makes sense that you would take damage whether you enter or exit, but (as written) the spell only says when entering. Am I missing something here?

Regardless of the wall being formed in a ring, when you move through the wall you are considering only the part of the wall you are moving through.

You are not "inside" the wall if you are standing inside the ring made by a wall of fire. You are only "inside" the wall if you are standing on the ring made by the wall of fire.

My 2¢ - 

As Farling says, you take damage when you enter the wall, not when you enter the area encircled by the wall. The wording is also "first time on a turn", not simply 'first time'.

If when the spell is cast you are in a square where the wall now exists, you take a dex saving throw and on a failure you take immediate 5d8 fire damage, or 1/2 that on a success.
Thereafter, if you stand your ground and end your turn still inside the wall itself, you take 5d8 fire damage at the end of your turn.
If you are standing on the hot side within 10' when it's cast, and you end your turn within 10' on the hot side, you take 5d8 fire damage.
If each turn you moved from one side of the wall to the other, you would take 5d8 damage every turn, because each time is the first time that turn.
If you enter the wall (again, not necessarily the area enclosed by the wall) and stay there you take 5d8 fire damage on the turn when you enter the wall.
Thereafter, if you end your turn inside the wall you take 5d8 fire damage at the end of the turn.
If in the course of a single turn you zigzag from one side of the wall to the other, you take 5d8 damage (i.e. for the first time on that turn that you entered the wall.)
If you move through the wall, then end your turn on the hot side within 10' of the wall, you take 5d8 fire damage. 
Thereafter, if you end your turn within 10' on the 'hot side' of the wall, you take 5d8 fire damage.

With a 20' diameter ringed wall, hot side in, you are bey definition within 10' of the wall if you are inside the circle.

The only thing that's just a tad ambiguous is whether you take damage if you are within 10' of the wall on the hotside, but not within the wall itself, when the spell is cast. I'd call that DM discretion because the spell wording is vague.

I think that about covers it.

According to the spell:

One side of the wall, selected by you when you cast this spell, deals 5d8 fire damage to each creature that ends its turn within 10 feet of that side or inside the wall. A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there. The other side of the wall deals no damage.

If the wall is a ring... and you are on the inside of the ring and within 10' (which everyone inside it is... since the diameter is 20'), then you take the damage.  When you pass through the wall, you take damage.  Once you are outside the ring, you no longer take damage.  Specifically it states "the other side of the wall deals NO damage".

Thanks guys, this makes total sense now.