Taken from Phineas and Ferb Wiki Forum: “Who is Ferb’s Real Mom and Phineas’s Real Dad?”
Let me get a few things straight here, (1) You do not know how much I wish this were true (2) Alright I’m 12, big whoop! I have given my idea great thought! And (3) Has it ever occurred to you that Candace is like 15 and Vanessa is 16 and whatever Dr. Doofenshmirtz did with whoever could have been a one-time thing… well, maybe two time thing. Okay here’s my story. If Vanessa is 16 and our time she would’ve been born in 1993 which would’ve been around the time he married Charlene. Now, he may have well, met Linda and she got pregnant with Candace. Then maybe about three years later he came back to Linda (they did not have to be married) and she had Phineas tough without him to see his son. Probably 15 years after his marriage he is divorced, I would say that it was his wife not him because he shows anger toward her and she seems rather neutral which to me indicates that she is sort of happy to be away but doesn’t really care either and well, Heinz may be angry because he typically is a very loving/caring person believe it or not, just look carefully, but he could be upset because he still loved her when it happened… ANYWAYS!! Candace and Doofenshmirtz wouldn’t have to have met because she would’ve been about three when he returned but Linda probably wouldn’t say anything about it now would it?
Now, here are the characteristics of Doofenshmirtz and Candace and Phineas share, 1. Talking, this Ferb does not do since he could never be possible related the Doofenshmirtz’s (2.) Phineas and Candace enjoy music. Ferb does to but he’s more instrumental rather than vocal… okay I would’ve come up with a third one but Candace is too hard to relate him too! Now he and Phineas both share the creativity (Dr. Doofenshmirtz is really creative, have you SEEN what he builds and how he plans to use it?!?!) Phineas and Doofenshmirtz both have that technology thing and mechanical work. They share similar hair (not colour wise but more shape).
And well, the next one I going to have to explain more, we all know how Dr. Doofenshmirtz was emotionally abused, neglected, and dozens of others but have you seen what he tries to do for his daughter? He, being unlike his own father, loves Vanessa very much and does what he can for her, he even likes Perry though he won’t admit to it, except for that one time, and he hasn’t tried to deliberately hurt Charlene has he? Yes, he ripped up a poster of her but he wouldn’t really hurt her!! He is a kind person at heart!! He shares that kindness thing with Phineas and well, sometimes Candace. And if you have noticed, he has a long-ish neck and Candace definitely has a long neck and Phineas is developing that little curve in his neck, not as severe as Doofenshmirtz but it’s still there. Doofenshmirtz is also described as short. Well, Phineas is definitely short and Candace was once too short to be ‘flawless girl’ so I’m thinking she’s not one of the tallest despite her huge neck!! Plus, both Phineas and Dr. Doofenshmirtz want to do something different each day.
Now, even if Candace had met Doofenshmirtz that would have been AT LEAST 13 years ago. And besides, he would’ve been in his twenties when Candace was born and think about it, he would’ve gone through his thirties and all the way up to early/mid 40’s (I just prefer to say that he’s about 42) So Candace wouldn’t have been able to recognize him at all. That’s pretty sad to think about… how could Disney allow this? ANYWAYS, well, that’s about all I’ve got. But hasn’t anyone wondered about Ferb? I guess you’re right. Whoever his mother was, was back in England.
Oh yeah, Vanessa and Candace wouldn’t have to of known each other and neither would Charlene. Doofenshmirtz is at least reasonable enough to know not to introduce Linda to Charlene. See, he marries Charlene. He meets Linda. Linda has Candace. Notice that this is all the while he has Vanessa and then after a period of time he gets back with Linda and she had Phineas. Now, he might have known about Candace but she wouldn’t have known him. That might also be one more thing that piles up on him and causes that dark patch under his eyes, a whole other family. Maybe that’s why he treats Vanessa so great (well, he loves her too but it might just be one more reason to love her). And he has been into the dating business though never quite successful… and that’s about all I’ve got left…