Mutagenic Growth

• WWizards of the Coast2011-06-01
  Official Ruling
  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.
• WWizards of the Coast2011-06-01
  Official Ruling
  To calculate the mana value of a card with Phyrexian mana symbols in its cost, count each Phyrexian mana symbol as 1.
• WWizards of the Coast2011-06-01
  Official Ruling
  Phyrexian mana is not a new color. Players can't produce Phyrexian mana.
• WWizards of the Coast2011-06-01
  Official Ruling
  If you're at 1 life or less, you can't pay 2 life.
• WWizards of the Coast2011-06-01
  Official Ruling
  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.

Rulings via Scryfall