Hiya!
(TL;DR Not a problem with a PC getting no XP. They make up for it by getting a bit more because of harder challenges to their level).
I'm with [MENTION=20564]Blue[/MENTION] (his first post on this thread, #3 point) and [MENTION=97077]iserith[/MENTION]. If the player misses the game session, then his PC either becomes an "APC" (no, not that, the A stands for Absent...mostly...). A character who is not able to attend, or misses some part of the game (beginning because they had to work late, or the end because they had to do something like pick someone up at the airport, etc), that persons character becomes an APC. While in "APC mode", the character generally is 'ignored' by the world if at all possible. The character hangs at the back and doesn't offer advice, doesn't perform skills, doesn't really do much of anything. If the situation requires the PC's interaction, one of the players makes the decision, with me overriding it if it seems out of character or isn't "necessary"; e.g., they find a chest and it's locked...the APC thief does NOT deal with it because opening the chest is not 'required'; when the player is back, then she can decide if she wants to look for traps and pick the lock).
With regard to combat, monsters will generally attack others first and the APC will try and do non-combat stuff (support or 'watch the hall' or something). But if combat is no option, a player rolls. The one exception to the rules is that the APC gets a free "delay of death rolls" until they can roll it; if the PC's can use some magic to keep the APC alive after the fight, then great. Yes, this does pretty much 'remove' the Death Roll mechanic from affecting the APC a lot of the time (PC's usually have SOMETHING they can do to help a character at 0 hp). EXCEPTION!: If the entire party is killed/removed...then an unconscious APC is automatically "captures" or (usually more likely) killed, resulting in a TPK. All of us understand that this is our rule. This is accepted by everyone at the table and we all think it is fine and makes sense. Yes, on occasion a player has left early only to find out between sessions "Er...sorry, Tracy, Silvia the Slick is dead. The whole party was killed by an otyugh". It is accepted as part of the game in our group.
Now, for PC's that can get to a safe spot (a camp, town, whatever), they are 'safe', because they aren't at the actual adventure (dungeon, etc).
The APC gets ZERO XP if they are 'safe' and not at the actual adventuring locale. Again, we all accept this as a group.
When/if a PC ends up lagging behind others...uh...well, never bothered us. Like, ever really. As I've said in other threads about this sort of topic, I've run 1e AD&D games where the PC's can be rather largely apart, level wise, from others. In other words, I can have a 7th, two 5th, a 4th, a 2nd and a 1st level PC all in a party (IIRC, AD&D 1e) and nothing "broke". Yes, the 1st and 2nd level PC's were hard pressed to not die...but that didn't detract from their players fun. In fact, it added an extra level of challenge and RP'ing opportunities as the 'newbs' were in awe at how amazingly heroic the 5th, let alone the 7th, level PC's were. And, with AD&D's XP system, the lower level PC's gained XP pretty dang fast so they were up to 3rd or even 4th in relatively short fashion.
In 5e, with the whole Bounded Accuracy thing...differently leveled PC's isn't much of a problem. The 7th level PC can take on a group of 5 goblins by himself while the 2nd and 1st can work together taking on two or three as a team. (actually, same sort of thing in 1e, but with a bit of a different "hit to damage taken" ratio). A lower level PC will and should get an XP bonus because the challenge is different for them (harder).
^_^
Paul L. Ming