What happens if a player has zero or less life but a replacement effect prevents them from losing?

State-based actions (CR 704.3, 704.5a) check whether a player has 0 or fewer life points and, if so, that player loses the game. However, this is a triggered outcome from a state: the actual 'loses the game' event must occur. Replacement effects (CR 614.1) can modify or replace that event before it happens.

The most famous example is the card Platinum Angel, which states 'You can't lose the game and your opponents can't win the game.' Under CR 614.1a, this is a continuous effect that replaces (prevents) the losing event entirely. When state-based actions would cause you to lose due to 0 or less life, the replacement effect intervenes and the loss simply does not happen.

Similarly, effects like Phyrexian Unlife replace the loss event by keeping you in the game (with poison counters used as a secondary loss condition instead). The key mechanic is that the player's life total can legally sit at 0 or below while they remain in the game, because the loss event itself is being replaced or prevented.

Concrete example: You are at -5 life and control Platinum Angel. State-based actions check your life total (CR 704.5a), see it is 0 or less, and would make you lose — but Platinum Angel's replacement effect prevents that loss from occurring. You remain in the game at -5 life until Platinum Angel leaves the battlefield.

Important caveat: if the replacement effect is removed (e.g., Platinum Angel is destroyed), state-based actions are checked again immediately, and you will lose the game at that point (CR 704.3).