Worldheart Phoenix — ruling 1

Casting Worldheart Phoenix from your graveyard by paying its alternative cost doesn't change when you can cast it. You can cast it only at the normal time you could cast a creature spell.

Worldheart Phoenix — ruling 2

If you cast Worldheart Phoenix this way, it is a spell and can be countered.

Worldheart Phoenix — ruling 3

As soon as you start to cast Worldheart Phoenix from your graveyard, the card moves onto the stack. At that point, it's too late for an opponent to prevent you from casting it by removing it from your graveyard.

Worldheart Phoenix : règles, fonctionnement et interactions (MTG)

Worldheart Phoenix 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 You may cast this card from your graveyard by paying rather than paying its mana cost. If you do, it enters with two +1/+1 counters on it.

How it works : Worldheart Phoenix

Original English text — Wizards of the Coast / Scryfall

#1February 1, 2009· Wizards of the Coast
Casting Worldheart Phoenix from your graveyard by paying its alternative cost doesn't change when you can cast it. You can cast it only at the normal time you could cast a creature spell.
No other card is named in this ruling.
#2February 1, 2009· Wizards of the Coast
If you cast Worldheart Phoenix this way, it is a spell and can be countered.
No other card is named in this ruling.
#3February 1, 2009· Wizards of the Coast
As soon as you start to cast Worldheart Phoenix from your graveyard, the card moves onto the stack. At that point, it's too late for an opponent to prevent you from casting it by removing it from your graveyard.
No other card is named in this ruling.