Snapcaster Mage has the ability to give an instant and sorcery card in your graveyard Flashback. When it does, the Flashback cost will be equal to the mana cost of the targeted card.
If Snapcaster Mage’s ability is used on a card with a Phyrexian mana symbol in its mana cost, then the Flashback cost will also have this Phyrexian mana cost. That mana symbol can then be paid for just as if it were cast from a player’s hand–by paying mana or by paying two life.
For example, if Snapcaster Mage’s enters-the-battlefield ability targets a Gut Shot in a graveyard, then this Gut shot will gain a Flashback cost of one red Phyrexian mana. It can now be cast by paying one red mana or two life. Keep in mind that it will be exiled when it resolves like any other spell cast via Flashback.