Ferb Fletcher is one of the protagonists of the Disney animated television show Phineas and Ferb, and also stars as one of the main characters in Phineas and Ferb The Movie: Across the 2nd Dimension. Ferb is indeed a playable character in Disney INFINITY. However, he has yet to be officially revealed. Ferb's existence was confirmed through gameplay videos both old and recent. However, Ferb does not appear in the Hall of Heroes, and it is unknown when he will be officially revealed. He's a likely candidate for a wave three release.
Ferb and Emperor Zurg have been found in the coding of Disney Infinity. Neither can do anything when spawned and played in the game however.