Do you gain XP based on your "level" or your XP total?

Question

Let's say a 3rd-level character with 5900 XP (100 away from reaching 4th level) clears two EL 5 encounters during an adventure. The PHB says on page 58 that XP is handed out at the end of each adventure, so this means they were in both of these encounters while they were 3rd level.

According to the table on page 38 of the DMG, a party of four 3rd-level characters gains 450 XP per character from an EL 5 encounter, whereas a party of four 4th-level characters gains 400 XP per character.

So, at the end of the above example, would the character have gained 900 XP (they were treated as a 3rd level character during the second encounter, because at the time of the encounter they had 3 HDs and the class features of a 3rd-level character, etc.) or 850 XP (they were treated as 4th level during the second encounter, because at the time of the encounter they had between 6000 and 9999 XP)?

Answer 1

Player Characters gain Experience Points according to their Effective Character Level at the time they overcome a challenge.

It doesn't matter how many XP they have, just what their ECL is. This is easier to visualize with an example:

Kurt, a human 1st-level fighter (ECL 1) with 0 XP at the moment, would gain 600 XP by defeating a vargouille (CR 2) by himself.

But if Kurt was turned into a vampire before starting the fight, and then defeated the vargouille with the help of his new vampire powers, he would only gain 225 XP. This is because Kurt would now have an ECL of 9 (the vampire template has +8 Level Adjustment), even though he still has 0 XP.

Experience Points are (in theory) rewarded according to the difficulty of the challenge, having more experience points doesn't make the encounter easier*, but having more levels usually does.

*Unless you're a desperate spellcaster using spells with XP components.

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Also, when to reward XP is up to the DM, he doesn't need to wait to the end of an adventure, he can award them at the end (or beginning) of a game session, or even mid-game if he considers it necessary. From DMG page 18:

You may want to award experience points at the end of each session, or you might wait until the end of each adventure. That’s up to you. However, the standard procedure is to give them out at the end of each session, so players whose characters go up a level have time to choose new spells, buy skills, and take care of other details related to level advancement.