I want ton know, if, because of the reach of the glaive, a PC with this weapon is in a flanking position (10 feet from the foe) with another PC who would be face to face with the foe (in an opposite side with the first PC)?
And has he had the right to do an attack of opportunity on a foe who would come in the threated zone (in the 5 feet)?
well, you are in melee and you are in reach to hit the enemy.
I would say yes.
I want to know if, because of the reach of the glaive, a PC with this weapon is in a flanking position (10 feet from the foe) with another PC who would be face to face with the foe (in an opposite side with the first PC)?
And has he had the right to do an attack of opportunity on a foe who would come in the threatened zone (in the 5 feet)?
Both you and the guy on the other side would be flanking.
If the foe provokes, you both may take an Attack of Opportunity.
If the foe takes a 5' step towards you, you may not take an AoO. (5' step does not provoke) If, while the foe is adjacent to you, he does something that provokes (casts a spell, for example) you cannot take an AoO with the glaive because he is inside your reach. If you have a spiked gauntlet, or armor spikes, or natural attacks or improved unarmed strike, you would still threaten at 5' and 10' and be able to AoO.
If the foe is farther away, and uses a normal move action to to become adjacent, you could take an AoO with the glaive when he attempts to move from 10' to adjacent (when he attempts to leave the threatened square). This AoO happens before he leaves the threatened square. If you use the AoO to trip him, he falls 10' away.
Thanks a lot ! That's exactly what I thought!