An attacking or blocking creature that phases out is removed from combat.

Permanents phase back in during their controller's untap step, immediately before that player untaps their permanents. Creatures that phase in this way are able to attack and pay a cost of during that turn. If a permanent had counters on it when it phased out, it will have those counters when it phases back in.

Choices made for permanents as they entered the battlefield are remembered when they phase in.

Any continuous effects with a "for as long as" duration, such as that of Mind Flayer, ignore phased-out objects. If ignoring those objects causes the effect's conditions to no longer be met, the duration will expire.

As a permanent is phased out, Auras and Equipment attached to it also phase out at the same time. Those Auras and Equipment will phase in at the same time that permanent does, and they'll phase in still attached to that permanent.

Phased out permanents are treated as though they don't exist. They can't be the target of spells or abilities, their static abilities have no effect on the game, their triggered abilities can't trigger, they can't attack or block, and so on.

Phasing out doesn't cause permanents to leave or re-enter the battlefield, so no "leaves the battlefield" or "enters the battlefield" abilities will trigger.