I guess so!

poor isabella... he really dosent have a clue..WAKE UP AND SMELL THE COFFEE PHINEAS!!!

If you observe carefully in "The Beak", and on some other episodes, you will see that phineas has trouble lying to Isabella. He scratches his ear or fidgets some other way. So he probably has feelings for her but like her he doesn't know if she will like him. I got that hunch from the Summer Solstice episode when Isabella mentions Ferb's map when confronting him on the island and he hugs her. I mean, would he do that to some other chick? He also seems to ask if Isabella needs a patch or whatnot and he will make the scene for getting the patch.

If only Phineas knew ... come on Isabella tell him!

MEGA SPOILER FOR MOVIE!
----------------------

They find out about Perry being an agent and at the end, for Perry to stay with them they decide to get their memories of the day erased. Here is what happens.
Isabella: Um...Major Monogram? So none of us will remember anything of today?'
M. Monagram: That's right.
Isabella: Good! *kisses Phineas*
Phineas: *big smile* Isabella!
Isabella: Hit it Carl!
Phineas: Wait! Wait! Wait!
Carl: erases memories
HE LIKES HER :) WATCH THE CLIP ON YOUTUBE TO SEE :)

I think he just sees her as his best friend ( except ferb ) so he probably doesnt know anything and when she kissed him he sounded very surprised so i think he hasnt almost got any clue

I do love her!

HE REALLY LOVES HER

She did kiss him (on the mouth :O) on the 2nd Dimension movie.
<3

I don't think so.Phineas is not interested in Isabella.He treats her as a good friend,not GF or something.