Phyrexian mana is not a new color. Players can't produce Phyrexian mana.

As you cast a spell or activate an activated ability with one or more Phyrexian mana symbols in its cost, you choose how to pay for each Phyrexian mana symbol at the same time you would choose modes or choose a value for X.

If you're at 1 life or less, you can't pay 2 life.

A card with Phyrexian mana symbols in its mana cost is each color that appears in that mana cost, regardless of how that cost may have been paid.

Cathedral Membrane's ability will trigger no matter why it's put into a graveyard during the combat phase, not just because of combat damage.

To calculate the mana value of a card with Phyrexian mana symbols in its cost, count each Phyrexian mana symbol as 1.