If a player has 0 or less life but has a replacement effect like Platinum Angel, do they lose immediately?

Normally, a player with 0 or fewer life points loses the game as a state-based action (CR 704.5a). State-based actions are checked continuously and would cause the loss immediately the next time they are checked — typically before any player gets priority.

However, Platinum Angel creates a replacement-style prevention effect: 'You can't lose the game and your opponents can't win the game.' This is a continuous effect that modifies the rules of the game for as long as Platinum Angel is on the battlefield (CR 614.1). Because of this effect, the state-based action that would cause the player to lose (CR 704.5a) simply does not apply — it checks, finds the player has 0 or less life, but the loss is prevented, so nothing happens.

The player continues to play normally at 0 or negative life. They can still take damage, draw cards, and take other actions. The only danger is if Platinum Angel leaves the battlefield — the moment it does, state-based actions are checked again and the player will lose immediately if still at 0 or less life.

Example: Alice is at -5 life but controls Platinum Angel. State-based actions are checked; the game sees she has 0 or less life but also sees Platinum Angel's effect, so she does not lose. On her opponent's next turn, the opponent destroys Platinum Angel. State-based actions are checked again immediately after it leaves — Alice still has -5 life and now nothing prevents the loss, so she loses the game right then.