In Phineas and Ferb, the story takes more focus on the Flynn-Fletcher Family, which consists of two separate families, Phineas's family and Ferb's family. Phineas's family (The Flynns) has Phineas, Candace, and Linda Flynn, their mother, while Ferb's family (The Fletchers) has Ferb and his father, Lawrence Flynn. You see, they're one family because Linda Flynn and Lawrence Fletcher married one day. The only thing we don't know... are Phineas and Ferb's missing biological parents.
I know that I shouldn't have said Phineas's biological Father and Ferb's biological Mother but I think it's too long to say that.
Anyway, if you notice, the other characters only have single parents as well (as in the other parent is unknown). But why am I most curious about Phineas and Ferb's missing parents? It's simple: they look a little different than the other characters. Phineas has a triangular head, while Ferb has green hair. I always wonder about who actually inherit those features to those children, especially Ferb's hair, since I haven't seen any children in the show who had technicolor hair.
In fact, many other people are also curious about their missing parents. Some even made theories on them. There are theories, which Phineas's father is Heinz Doofenshmirtz, because their similar triangular heads and sprouts, also their high intelligence and hand coordination. This theory is rather famous in the Phineas and Ferb fandom. In my opinion, this could be nearly false, since during the Doofenshmirtz Family Reunion, I could not see any singular person who, even nearly, look like Phineas. This blog post to me is the most possible theory, which explains that Perry the Platypus is his father who does not want to be known... Perry (if he is Phineas's father) would have problems with his relationships with Linda, since he mostly works and less cared for Candace and Phineas. After he was turned into a platypus, he makes up to it by always guarding Phineas, Ferb, and Candace, and the rest of the Flynn-Fletcher Family.
Now about Ferb's mother. Most fans were guessing that he was an alien. Well, until "Invasion of the Ferb Snatchers", of course. But that's because there's no possible proof yet. And since he looks like his grandfather Reginald, Lawrence is clearly to be Ferb's biological father. So maybe... Ferb's mother is an alien. I've one put a Wild Mass Guessing in TV Tropes that he was born ala The Sims 2 Alien Pregnancy, but it's actually up to you.
My hopes is that they (Dan and Swampy) would finally reveal Phineas and Ferb's missing parents during their movie, "Phineas and Ferb: Across the Second Dimension". I guess I'm done writing this blog post. Thanks for reading and comment too if you like!