A phased-out permanent is explicitly treated as though it does not exist for almost all game purposes. CR 702.31d states that while a permanent is phased out, it is treated as though it does not exist, meaning other players and game rules cannot 'see' it during that time.
State-based actions (CR 704) are the game's automatic checks that happen continuously — things like checking if a creature has lethal damage, if a player has 0 life, or if an aura is unattached. Because a phased-out permanent is invisible to the game, it is not evaluated by state-based actions while phased out. It cannot die from damage, be destroyed for losing its legal enchantment target, or be checked for any SBA condition.
There is one important nuance: auras and Equipment that are phased out with a creature (i.e., they phased out as a consequence of the creature phasing out) remain attached to that creature while both are phased out. This is tracked implicitly, and when the creature phases back in, those permanents phase in with it already attached — no SBA check destroys the aura while everything is phased out together.
Concrete example: You control a 1/1 creature with three -1/-1 counters on it that would normally die from state-based actions (0 toughness). If somehow it phases out before SBAs are checked, it is ignored — the SBA does not apply while it is phased out. When it phases back in, SBAs will check again and it will die at that point.
The relevant rules are CR 702.31d (phasing and 'treated as though it doesn't exist') and CR 704.1–704.5 (state-based actions and when they apply).
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