If you’re at 0 or less life and Phyrexian Unlife leaves the battlefield, you’ll lose the game (unless some other effect is keeping you from losing).

You can still lose the game for other reasons, including having ten or more poison counters or drawing a card from a library with no cards in it.

If you’re at 0 or less life, you can’t pay any amount of life except 0.

Phyrexian Unlife won’t affect damage that reduces your life total from a positive number to 0 or less. For example, if you’re at 3 life and are dealt 5 damage, you’ll end up at -2 life. The next time you’re dealt damage, it will be dealt as though its source had infect.