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http://dogman15.deviantart.com/art/Phineas-and-Ferb-theme-song-158056539
no that song was made recently and the song would be on a Phineas and ferb soundtrack CD
yes they did do the theme song.
They say for summer vaccation in the theme song!
"Locating Frankenstein's brain"
Well, I know it is by Bowling for Soup on the Phineas and Ferb Soundtrack. I beileve it is called Gotta make Summer Last or 104 days
it is from the phineas and ferb christmas special.
phineas and ferb musical cliptastic countdown
its the one when Phineas and ferb direct the play "the princess monster". NOOOOOOOOOOOOOO!!!! Its the episode where Phineas and ferb sing the song gitchi gitch goo and when Candice happens to be the 100th contestent and she sings the song with them...sooooo coincidently, or serendipitously, it became a brother sister thing. :)
So first you have the opening, and the totally first song in the show is ..........Perry the Platypus Theme Song - in episode Rollercoaster
If you are talking about the theme song, no, but she does sing 'That Christmas Feeling' (Which i think is the song you meant)
Melanty Weleanor sings "The Only Fish For Me" from Phineas and Ferb.