Can you activate Sun Disc after casting a burn spell but before Immortal Phoenix revives from the graveyard?

Ruling: No, you cannot activate Sun Disc after casting the burn spell but before Phoenix revives. Sun Disc is not a reaction, so you must activate it before casting the spell.

Sequence:

• Play your first card (or Sun Disc itself counts as the first card on the turn it's played)
• Tap Sun Disc to give a unit +2/+2
• Play the burn spell (as your second card)
• Phoenix triggers and enters play ready

Nuances:

• Sun Disc counts as a card played on the turn it enters play, allowing you to tap it immediately that turn