Trivia: Phineas and Ferb The Movie: Across the 2nd Dimension
All There in the Manual: The official movie magazine features a comic showing how Doof 2 transformed Perry 2 into Platyborg, and took over his Tri-State Area, although its canonical status may be in question.
What Could Have Been: "Mysterious Force" was a song originally made for the movie, sung from Candace's point of view. Unfortunately, due to scaling back of the movie, it was scrapped. (It's on the soundtrack if it means anything)
According to Word Of God in the orginal ending, it turns out the 1st dimension isn't REALLY the first dimension, but rather another one, different from the regular episodes. So then nothing canonically happened and then they said that universe's Phineas and Isabella would hook it up. They scraped it because they didn't want to piss off the fans.
Essentially all the deleted scenes. We not only have "Mysterious Force", but also Vanessa (from both dimensions) being in the film, an explanation for why the entire population of Danville didn't need their memories wiped*
the robot invasion was passed off as a weather balloon