If you kill Immortal Phoenix with a spell and you then replay him?

Yes, you can replay Immortal Phoenix from your trash if you kill it with your own spell.

According to Rule 383.2.c.1 and the associated FAQ, this interaction works because the Phoenix is moved to the trash during the resolution of the spell, and its trigger condition is evaluated immediately after the unit is killed.

The Sequence

1. Play the Spell: You play a spell that includes a "Kill" instruction (e.g., Cull the Weak).
2. Resolve the Spell: As the spell resolves, you choose to kill your Immortal Phoenix.
3. Move to Trash: The Phoenix is moved from the board to the trash (Rule 415.1).
4. Trigger Evaluation: The spell finishes resolving. The game then checks for triggered abilities. Because the Phoenix is now in your trash and you were responsible for killing a unit with a spell, its ability triggers.
5. Play from Trash: You may now choose to pay the cost ([1] Energy and [1] Fury Power) to play the Immortal Phoenix from your trash to your base or a battlefield you control (as per FAQ #4527).

Important Nuances

• Responsibility: You must be the one who controls the spell and is responsible for the kill action. If an opponent's spell kills your Phoenix, you cannot use its ability (FAQ #6480).
• Timing: You cannot react to the unit being killed while the spell is still resolving. The ability triggers and is placed on the chain only after the spell has finished resolving (Rule 157.3.a).
• Placement: When played from the trash via this ability, the Phoenix enters the board exhausted (FAQ #4527).