Concentration check and AoO

[alex1g]

So here's the scenario. I have a player that moved into a threat range. On his turn he gets in AoO doing 21 damage. Tell him to make concentration check, he tells me that he doesn't have to.
I'm like why?
He tells me that the concentration check is made on the movement and not for the round that includes of casting a spell or doing a psionic check and movement.
Movement and Standard is a round and it includes the actions made in that round.

VS player response

the AoO was from the movement phase. (before he even cast anything.) the move through threatened spaces triggers an AoO. so you whacked him for moving into a space. once he arrived at said space (after you whacked him) he then begins the process of casting/manifesting. at this point you can choose to take another AoO if you have the ability to do so if you want to to try and prevent the spell "as it is being cast" by dealing damage.
the SRD and PHB explains this under concentration: "during the action" not "during the turn".

[Kish]

Okay, let me see if I can sort out the situation and what you're asking.

Scenario: The PC moves up to an ogre, who has reach because of being Large. The ogre takes an attack of opportunity as the PC goes through his threatened area, to get from 5 feet away to adjacent. Then, while adjacent to the ogre, the PC starts to cast a spell. This does provoke a second attack of opportunity, but the ogre needs Combat Reflexes or something similar that gives him more than one attack of opportunity per round to be able to take it. If the ogre takes a second attack of opportunity and hits, the PC needs to make a Concentration check for that. Not for the damage taken while moving.

[ExLibrisMortis]

You can take a bazillion damage up to the moment you start casting, and it won't affect your ability to cast. You can only be interrupted during the actual casting of the spell, which is from the moment the first casting action is taken until the moment the spell's effect begins.

[Mehangel]

While likely not relevant to this scenario, sometimes a creature takes 'continuous damage' which applies to any concentration checks made that round.