We had a situation where a player wasn't present and the other players played his character in a way typical for him, and he was unfortunately killed by a cone of cold.  Everyone felt a bit bad about this and as a group discussed how to handle players not being present.  We resolved that a player could decide if he wants he character played actively by one of the other players and accept all associated risks, and be rewarded full experience,  or his character is played very cautiously/passively, avoiding conflict as best as possible, and not receive experience for the session.  Everyone seems happy with the system.  I don't have a problem running a campaign with characters of different levels, I think it's fun for a slightly lower level character to be more cautious and strategic.  

Quote from Nat_30 >>

In my group, you don't get XP if you aren't at the session.

However, we also have the rule that the lowest-level character in the party cannot be more than two levels behind the highest-level character in the party. So if Sam isn't able to attend the game for several months due to grad school, and when they return the highest level in the party is level 9, Sam would be able to auto-advance their level 3 character to level 7. That way they can still keep up with the rest of the party during encounters.

I think this method allows us to reward attendance while also preventing too stark of an imbalance between characters. Best of both worlds!

 I use similar.  Works very well!