In "Summer Belongs to You" Phineas is shown in a flashback having his bike stolen from him kwa Buford, but in other episodes that took place before this one, both Phineas and Ferb are shown with their bikes. So do wewe think they bought new bikes?
Possibly yes. I've seen them in various episodes with their bikes, and each time they're a different color. It is a bit confusing I must say, but sometimes the creators cannot remember everything and often forget, which resolves into them throwing things in to their animation, forgetting the process and other things they have said.
Basically, I am not sure if it is yes au no, possibly a maybe. :3 Buford has stolen their bikes, and yes, Phineas and Ferb have been seen with their bikes before that, like in "Traffic Cam Caper", when Phineas and Ferb installed the rocket to their bikes to get the CD back from Norm. I've seen their bikes in many other episodes, so clearly the answer is still, like I said, a maybe.