Catch Phrase: Many, almost all lampshaded or subverted at some point. Often they'll be borrowed by another character, most obviously in "Hail Doofania!":
"I know what we're gonna do today!"
"Whatcha doin'?"
Isabella is apparently very protective of this one, as evidenced during "Out of Toon" when she interrupts Buford saying it with a sharp elbow to the gut (Buford appears to fear her wrath and apologises, but she shuts him up and say "We'll talk about this later"), and the Christmas Special, where Candace says it and we get a cut to Isabella making an "I sense a disturbance" face.
In "Suddenly Suzy", Isabella is visibly aggravated when first Suzy then Candace steal her "Whatcha doin'?" thunder. "Do I even need to be here?!"
"Hey, where's Perry?"
"Aren't you a little young to be a [insert incredible occupation here]?" "Why yes. Yes I am."
"Ah, Perry the Platypus, your timing is incredible, and by 'incredible' I mean COMPLETELY CREDIBLE!"
"CURSE YOU, PERRY THE PLATYPUS!"
"Oh, there you are, Perry!"
"Oh, there you are, Agent P!"
"Mom! Phineas and Ferb are [insert Cassandra Truth here]!"
"Mom! Mom! Mom! Mom..."
"You've gotta see this!"
"He's a platypus. They don't do much."
"So kids, want some snacks?"(or some variation in place of snacks, often pie or cookies)
"Hey, is that [insert band here]?"
Khaka Peü Peü, thank you very much.
"In retrospect, I question the wisdom of..." An irregular one that Doofenshmirtz, Phineas or Ferb share.
Candace's evil/manic laugh.
"[Insert description of person here] [Insert name of person here] is out. Peace!"
"Buh-buh-buh-but! Buh-buh-buh-but!"
"(in response to someone's reply that what they said didn't make sense) It doesn't have to, I've got a monster truck!"
Candace and her mom's most commonly used banter:
Candace: Ok, tell me you don't see a (unbelieveable device) in the backyard! Linda: Ok, I don't see a (unbelieveable device) in the backyard.
"Will you keep it down? I'm trying to use the [Insert form of communication here]!
"Why would you open a (insert sales product here) store without any (previously mentioned products)? Did you think (said products) would just fall out of the sky?!"; Hasn't been used lately, though.
Normally used from time to time:
Any person, usually Candace: [Shows them Phineas and Ferb's creation and makes a rather unflattering comment about it]
(Any of Doof's inventions makes the item disappear/look good instead)
Linda: That wasn't very nice.
A dialogue that happens whenever Doof recognizes Perry:
Doofenshmirtz: A platypus?
(Perry puts on his fedora)
Doofenshmirtz: PERRY THE PLATYPUS?!
Candace: "Stacy? I'm gonna have to call you back."
Also, Stacy and Candace's girl screams back and forth over the phone, usually in response to something Jeremy does or says.
"This is the best day ever!" said by Candace whenever something good happens to her.
"Great googly-moogly!" said by Major Monogram whenever he gets surprised.