It ain't 'cause he plays better with his back against the wall ...
"I think it pays better!"
PHIL'S GOT GAMBLING JOKES!!
But seriously, 48-year-old Lefty tells TMZ Sports he officially started his training Thursday for the day-after-Thanksgiving, pay-per-view battle ... explaining, "I gave myself a month to get ready for it."
It's not just about fattening his bank account, Phil says he's REALLY excited about the money the event will raise for charity.
Phil says some of the money will go to the Children of the 58 fund -- which will provide scholarships to the children of the 58 people killed in the 2017 Las Vegas shooting.
Another portion will go to Birdies for the Brave -- a foundation started by Phil and his wife -- dedicated to supporting wounded military vets.
Props to Phil ... now go hit the range and get ready -- Tiger's waitin'!