The series never says, but there are many fan theories. Most of them think that Dr. Doofenshmirtz is their real father. Here are some theories related with with their real father: http://disappearedflynn.wordpress.com/
in one of Dr. Doofenshmirtz flash backs the one where the phineas and ferb reverse engineer Dr. Doofenshmirtz inventions he was dating Linda (Phineas's blood mom) so that might mean that he had Phineas with her.
Doofensmirtz but it wont be released until the final episode.
If Doof is their dad that would be gross because remember Ferb has a crush on Vanessa... :X
Doctor Heinz Doofensmirtz is Phineas's real dad! (Not really).
There real father nobody knows but I think it may be Dr.D
we don't know because it has never been in any episodes. but i think i read somewhere that Jeff March and Dan povenmire didn't 'create' a dad for phineas or a mum for ferb
Ferb is Phineas' step brother because Ferb Fletcher is the real son of Lawrence Fletcher, who is Phineas' step father. Candace Flynn and Phineas Flynn are the real sons of Linda Flynn Fletcher.
Ferb is Phineas' step brother and the Dad's real son. Phineas and Candace are the Mom's real kids and the Mom and Dad got married when all the kids were small. The dad's side of the family is from England and are in a couple of episodes and the mom's parents are in a couple too. The mom's parents are in the episode were they go camping.
Ferb is the stepbrother of candace.Phineas and candace are real siblings.
Well , no one really knows. She lives across from Phineas and Ferb (Whose real name is Frank)
weird, demented looking people
Ferb is Phineas' step brother because Ferb Fletcher is the real son of Lawrence Fletcher, who is Phineas' step father. Candace Flynn and Phineas Flynn are the real sons of Linda Flynn Fletcher.
There Father's Name Is Lawrence.
Phineas and Ferb - 2007 Phineas and Ferb Interrupted A Real Boy 3-8 was released on: USA: 15 July 2011
His full name is Phineas Flynn.
sadly they are not real cuz phineas is cute
Ferb is Phineas' step brother and the Dad's real son. Phineas and Candace are the Mom's real kids and the Mom and Dad got married when all the kids were small. The dad's side of the family is from England and are in a couple of episodes and the mom's parents are in a couple too. The mom's parents are in the episode were they go camping.
Phineas and Ferb got busted for real tons of times.
Ferb is the stepbrother of candace.Phineas and candace are real siblings.
Well , no one really knows. She lives across from Phineas and Ferb (Whose real name is Frank)
It is just Candace The Complete Name is Candace Katherine Flynn
a real boy
Only on Phineas and Ferb. :)