The Real Dad Theory
Doofenshmirtz is Phineas's father. This is mostly based around the similar head shape, but leads to several other ideas. Firstly, it explains why Phineas is so intelligent being the son of a scientist, also they appear to have infinite funds, possibly because of a trust fund or similar that Phineas has access to. Phineas also appears to have little to no care towards any side effect caused from their creations, the giant rollercoaster that goes through town would have caused multiple crashes, they carved Candace's face into Mount Rushmore, they make a time machine and destroy the timeline, they hold a medieval jousting tournament, they build an escalator to THE MOON, and that's just the first half of season 1. This is possibly a hereditary defect from Doofenshmirtz. Also: Perry is solely to prevent the meeting of father and son. 2 geniuses, especially a very mould able young mind. Could do some very serious damage.Dat bo Phineas look like a Dorito. And Ferb look like a block of cheese.
The Imagination Theory
The Phineas And Ferb Theory says that (like ) the real Candace has lost her mind due to bad things happening in her life.
The theory says that Phineas was her little brother and he was bullied a lot in school because the other kids thought he was too nerdy. One day a bully punched Phineas in the head really hard and it killed him.
That day changed Candace forever.
Candace couldn’t deal with Phineas’ death so she built an imaginary world where he was still alive and playing with her other brother Ferb. Ferb has a disability and can’t talk or do much, but she imagines him and Phineas going on great adventures together.
The real reason why her mother can’t see the things that Phineas and Ferb build is that, well, they simply aren’t there. And the more Candace tries to convince her mother the more crazy she becomes.
The villain of the show Dr. Doofenshmirtz is actually Candace’s psychologist who she’s been seeing twice a week. It started to put a strain on the family to try and pay for all these expensive doctor visits though. In desperation her mother had an idea to write down all the fantastical stories Candace was telling her and try to sell it to Disney as a show, and it worked.
The biggest bit of evidence that goes into this theory is in S2E33, Wizard of Odd, where Candace goes into a parody of the Wizard of Oz with Doof as the Wicked Witch. One of Phineas' lines is "I mean, it is Candace's dream". This means that Candace knows about Doof and his evil intentions, because of which it could be inferred that Candace made Dr. Doofenshmirtz, Perry and their rivalry up.
So that’s The Phineas And Ferb Theory, but is any of it true.