Yes, they did.
Yes. They will get married.
yes with Isabella
probably it's revealed in "Phineas and Ferb's Quantum Boogaloo", an episode in season 2
As far as the series has gone, Isabella was never Phineas's girlfriend.
YES
Yes, yes they do in 'Summer Belongs to You' when they are stranded on an island and Isabella tells Phineas not to give up.
It was not specified in the episode "Phineas and Ferb's Quantum Boogaloo."
No. Phineas has no interest in Isabella, however Isabella has a HUGE crush on Phineas
Phineas has a Crush on Her. Phineas Asks Isabella if she Wants to go on a Romantic Cruise. And he takes her hand through a haunted House (One good scare oughta do some Good) And Phineas Flirts with Isabella and Isabella Flirts Back. (The Beak)
Because Isabella loves Phineas!
Ummm... I don't think that there's a very good chance of Phineas and Ferb getting hitched. According to the episode "Quantum Boogaloo", Isabella will probably get married to either Phineas or Ferb.
I'm pretty sure there is going to be a episode where they get married i don't really know.