Phoenix of Iteration is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Opus — Whenever you cast an instant or sorcery spell, Phoenix of Iteration perpetually gets +1/+1. If five or more mana was spent to cast that spell, you may exile Phoenix of Iteration, then return it to the battlefield tapped under its owner's control. This ability also triggers if this card is in your graveyard.