Is Slushy Burger a reference to any restaurant chain?

Mr.Slushy Burger or (Mr.Slushy Dawg) is a frequently occurring restaurant chain in Phineas and Ferb. Jeremy is shown as working there. Candace also tried to get a part time job in Unfair Science fair.

Now, I was curious if this is a reference to ant famous restaurant. In Bully Code, the motto of the restaurant is shown.

Baljeet: Do you find it as curious as I do that your menu has not changed since you opened in 1929?

Jeremy: I know! It's awesome, right? It's our motto.

Slushy Dawgs Will Never Get Any Better.

^{From The Bully Code transcript.}

This sounds familiar to Wendy's. Their slogans have changed from time to time. Some of them are

It's better here
It's way better than fast food... It's Wendy's

Is Mr. Slushy Burger related to any restaurant chain or in particular Wendy's? Any interview or some other source which says this or against this?

reference phineas-and-ferb

Best Answer

It sounds like it is a reference to a bunch of fastfood-(related) chains.

Slushy the Clown is the mascot to Slushy Burger, which seems to be a reference to McDonald's:

Mr. Slushy Dawg may be a reference to Slush Puppy, a popular brand of slush in fastfood restaurants.
The clown resembles the Big Boy mascot with his signature "holding the hamburger in hand" pose: