Here it is, the Phineas and Ferb Theory:
The events taking place in the show are all the delusions of a group of kids in a children's psychiatric facility.
That's why the parents never see anything and why everything is so easy to do in that world. In reality, Phineas and Ferb's inventions are made of toys. The reason nobody ever gets hurt on anything is because (since they're locked up) the toys their inventions are made from are childproof to prevent their use as a weapon. The reason they "disappear" at the end of every day around the same time is because they have scheduled time to play with them.
The reason the parents go away so often during the episodes is because, while they visit as often as possible, they aren't allowed to be with their children all day. The kid's minds make the grocery excuse to explain why the adults are so often absent.
Phineas is Schitzophrenic and detached from reality. He's so happy all the time because of the mood stabilizer's he has to take to keep him from becoming depressed and suicidal. The 3 subplots that run in the series are all his mind's interperetation of the everyday occurances that go on around him.
Ferb is a nonverbal Autistic whose social withdrawal got so bad that he couldn't go to normal schools, because he would have a meltdown if he did. He inexplicably became attached to Phineas. In their world, Ferb chooses not to speak but can still mantain healthy relationships as he was unable to in real life. His kinship with Phineas is the reason why they're "brothers" without being related.
Isabella has an unnamed psychotic break from reality, caused by abuse from her father (which is why he's never seen) and the reason her mother is so friendly and motherly is to compensate for her real mom's inability to protect her. Her obsession with Phineas is caused by Reactive Attachment Disorder.
Candace has multiple personalities, including Perry, which is why she and Perry are so deeply connected and why Perry experienced Candace's dream. This is why her moods cycle so much. Jeremy, Stacey and Jenny are all her personalities, which is why Stacey was her "eyes, ears and mouth" in "Put That Putter Away!", and it's also why she acts so differently when they're around. The other children recognize Candace's personalities as different people as well and treat them as such. Candace's mom's frustration with her busting the kids isn't even about the busting, but instead about her giving into Phineas's delusions and being so aggressive to her counselors.
Buford has bipolar disorder, which is why he goes from deep and sensitive to aggressive and spiteful so often.
Baljeet is Obsessive Compulsive.
Dr. Doofenshmirtz is a Doctor at the facility who, while not a bad person, is unliked severely by Candace. When she's in her Perry personality she's more prone to break rules, which is why Doof "traps" her so often and why he only fights her if she attacks first.
Carl is an orphan who had nowhere else to go. He got involved in the kid's games once (in Undercover Carl) and was still in reality, which is why he was so reluctant to play along. He grew attached to Monogram, since he was the only adult who was ever really kind to him.
Major Monogram is Carl's social worker who interacts with the kids from time to time. Candace has an unexplained fondness for him and when in her "Perry" personality sees him as a part of their separate reality as his boss, since he always tries to encourage "him" to fight the illness. What Phineas and the other kids involved see as him bossing Carl around is simply his tough-but-fair way of trying to help the boy have a future.
Django was depressed, but after connecting with the boys, he was checked out by his dad, although he's relapsed once.
The Fireside Girls are Isabella's friends from her old life who come to visit often and play along with her delusions.
The recurring background characters are random Doctors, visitors, social workers and other patients. The "did you think it was just going to fall out of the sky?" couple are the parents of a patient who are always fighting about their kid's illness and the father is usually right. The extras from the first episode were kids who were almost done with treatment and were checked out soon after.
This also explains why it's perpetually summer, why the kids always wear the same clothes, why nobody goes to school and why none of the adults ever see what the children do.