I found on Wikipedia:
"On January 11, 2011, Disney Channel Worldwide chief Gary Marsh announced that an animated/live-action feature film based on Phineas and Ferb was in development by Tron: Legacy producer Sean Bailey. Series creators Dan & Swampy are in the early stages of writing the film, for a release in 2013. In an interview with Jim Hill, he revealed that Disney's idea for the movie would include the beginning and the ending of the movie being animated, but the middle portion having the characters portrayed by actors. However, the movie wouldn't be as big-budgeted as a Pixar film, would be released late in summer, and would include at least one popular actor, like Jim Carrey, as Doofenshmirtz, to get parents interested in it."