The 36-year old produced the sole clear on the 13-year-old grey gelding, with the later drawn Trevor Breen (Adventure De Kannan) and William Funnell (Dorada) having to settle for second place with four faults apiece, both knocking down the open ditch, near to the end of the course.
Last to go Tina Fletcher looked to be having a super round on Promised Land, the exuberant winner of the class in 2011, but the gelding stumbled slightly at the bottom of the bank and unseated his rider, clearing the way for Phillip to take home the £36,630 first prize.
"I never thought I'd be leaving Hickstead this week as the winner of the Derby," said a delighted Phillip. "It's an amazing achievement and it's the one everyone wants to win.
"To be honest I was worried about all of the fences. But the Devil's Dyke was my biggest concern, as we had four faults there last year, so once I jumped that I had to focus on not getting time faults. We had a rattle at the open ditch and then I could hear the crowd cheering, so I knew I was in with a chance."
Phillip is based at Greenacres Equestrian, a riding school owned and run by Di and Pennie Cornish. He has produced a number of top showjumpers, including Talan and Je T'aime Flamenco, but most have then been passed on to other riders to continue their international careers.
"'Carter' is the first horse I've kept on and he's got me to where I am now. I got him as a three-year-old and he wasn't gelded until he was eight and he was so rudely behaved."
Phillip learned to ride at Greenacres as a child, and still mucks out every day. "I didn't get any financial help from my parents growing up. I started riding at Pennie's 29 years ago and I've worked my way up," he said.
Trevor Breen, who finished joint-third last year with Adventure De Kannan, was gracious in defeat. "I'm genuinely happy for Phil - he is a great friend of mine," he said. "But I came here to win, not to make up the numbers.
"Addy has been on unbelievable form this year, then he had the trauma of the eye operation [he had his eye removed]. This is only his third show since then, so to come back like this is incredible - I can't ask anymore of him. To win would have been a fairytale, but it wasn't to be. There always seems to be one fence catching me. It's been a tough week for me," he said, referring to the death of his mother Mary Breen.
William Funnell found history repeating itself when he was runner-up for the second year in a row with the 15-year-old Dorada.
"I know how good it feels to win, and Phil is a real trier and he deserves it," said William, who won the class in 2006, 2008 and 2009. "He's a nice guy so if I've got to get beaten it's good to be beaten by him."