If Hextech Ray is used to kill a Pouty Poro, will this trigger Immortal Phoenix's ability?

Ruling: Yes, Immortal Phoenix will trigger. The source of the damage that killed the Poro is the Hextech Ray, so it's considered to have killed it.

Nuances:

• The game does not treat this as state-based actions killing the unit; the spell itself is considered the source of the kill.