Pinhead Pierre was the host of a kids' television program that aired in Great Britain called The Pinhead Pierre Show!.
He was a favorite of Lawrence Fletcher. His show also inspired Phineas to make a superhero cartoon, "Team Improbable". Before the show starts, Phineas dresses like Pinhead in order to introduce the show. This might mean that Phineas really liked the show and wanted to include Pinhead's likeness because of the inspiration it gave him.
Pierre has been in the background of two episodes, with a normal head, and no giant costume (suggesting his "giant" head is actually a camera trick utilizing an over-sized suit and forced perspective).
His appearance is described in the show's title song. The song says that he "has the smallest head of which I'm aware, and it's about the size and shape of a pear". Who this "I" is referring to is currently unknown, but it could mean the singer/singers of his show's theme song.
After this episode, Pinhead Pierre can be seen in "Rollercoaster: The Musical!" and later makes an appearance in "Magic Carpet Ride," inspiring again Phineas and Ferb to reenact one of his episodes by creating a real magic carpet.
In Phineas and Ferb The Movie: Across the 2nd Dimension, Holly is seen wearing his uniform while riding the flying carpet during the robot battle.