Joe Robbins says Phillips suffered a stroke before dying early Friday morning at a Memphis hospital. He was 40 years old.
The Commercial Appeal reports Phillips recorded the well-regarded solo album "Da Devil's Playground" in 1999. He left Three 6 Mafia the following year.
More recently Phillips reunited with other former Three 6 Mafia members to form Da Mafia 6ix.
Da Mafia 6ix member and producer "DJ Paul" Beauregard said in a statement to the newspaper, "Everyone who knew Koop knew he was very funny and the coolest guy ever. He will be missed. We made classics together that'll live on."
Funeral arrangements are pending.