Pressure treated wood is a bit of a misnomer these days. Unless the post is incised (little grooves running along its length) it is probably just dipped in a solution that helps retard fungal growth and gives it a neutral and consistent colour. Encasing this wood or any wood for the matter in concrete will accelerate the decay process. Concrete retains moisture and moisture is an enemy of wood...even a small concrete collar poured around the post at grade will eventually cause the wood to rot. In my experience using granular in the post hole base and tightly packing screening at the top of the hole will keep the post in place and allow the water to run away from the post.