Why does Phineas from the shoe and Ferb have a triangle head?

Phineas from Phineas and Ferb does not have a triangle head. He has a round head, just like most cartoon characters. It's likely you're thinking of a different character, or perhaps a fan-made illustration that depicts Phineas with a different head shape.

If you can remember any other details about the character or image you're thinking of, I might be able to help you identify it!