Historical Rules · 1 May 2011
Rule 107.4f
Official source text from the 1 May 2011 Rules release
A Phyrexian mana symbol represents a cost that can be paid either with one mana of its color or by paying 2 life. Example: {W/P}{W/P} can be paid by spending {W}{W}, by spending {W} and paying 2 life, or by paying 4 life.