Does a creature that phase out count as leaving the battlefield?

When a permanent phases out, it is not considered to have left the battlefield. Comprehensive Rule 702.26d states that phasing out does not cause any 'leaves the battlefield' triggered abilities to trigger, and the permanent simply becomes treated as though it doesn't exist temporarily.

This means several important things: 'enters the battlefield' abilities will not trigger when the permanent phases back in, auras and equipment attached to it stay attached (they also phase out with it), and counters on it are preserved. The permanent's owner doesn't lose control of it, and it just becomes invisible to the game until it phases back in.

CR 702.26e further clarifies that continuous effects that would end when the permanent 'leaves the battlefield' do NOT end when it phases out, because phasing out is explicitly not a 'leaves the battlefield' event.

Concrete example: You control a Serra Angel with a +1/+1 counter on it and it phases out due to an effect. No 'leaves the battlefield' triggers fire. When it phases back in on your next untap step, it still has its +1/+1 counter, and any auras attached to it come back too — no 'enters the battlefield' triggers fire either.

So phasing is intentionally designed to be a 'pause' on the permanent's existence rather than a true zone change, making it distinct from blinking or bouncing a permanent.