Phineas decided to propose to Isabella and Ferb had to stick around for the proposal because he was happy for his brother. Phineas went to Isabella and here's what they said during the proposal:
Isabella: Yes, Phineas.
Phineas: I have to talk to you.
Phineas: I know we've been together for a couple of years and I thought this would be the right time.
Isabella: What do you mean?
Phineas: Isabella.... Will you marry me?
Isabella (crying): YES, YES I WILL MARRY YOU!
Isabella kissed Phineas for a long, long time and didn't stop. Ferb, his friends and Candace screamed, "YES!" like banana's. Phineas and Isabella got married and Isabella gave birth to two children and Phineas called them Matthew and Donald. They lived happily ever after forever.
EXCEPT FOR THOSE 104 DAYS OF THEIR SUMMER!