The two-month grace period covers all types of visas, including tourist and expired residency permits
The first major of the Covid-19 era, played without fans on a cypress-lined municipal course, looked headed for a playoff as the day began with 11 players within three shots of Johnson, and at one point late in the round seven players shared the lead.
Then, after narrowly missing a birdie at 15, Morikawa hit the shot of the tournament as his drive at the 294-yard par-four 16th settled seven feet from the cup to leave him with a putt for eagle.
"We were just hoping for a really good bounce, and we got it, hit a really good putt, and now we're here," said Morikawa, who collected the third win of his career and the second since the PGA Tour resumed in June.
With the win, Morikawa joins Jack Nicklaus, Tiger Woods and Rory McIlroy as the only players since World War Two to win the PGA Championship at age 23 or younger.
"You know ... wasn't meant to be," said Koepka. "Three in a row, you're not really supposed to do two in a row looking at history, but that's all right. Got two more (majors) the rest of the season and we'll figure it out from there."
Koepka finished on three under for the tournament, tied for 29th.
Johnson, who was in a share of the lead late in the day, suffered a bogey at 14 shortly after Morikawa made his move. He is the first player to finish runner-up at the PGA Championship in consecutive years since Nicklaus (1964, 1965).
Casey was tied for the lead when he reached the 17th but his hopes of a maiden major championship suddenly slipped away as Morikawa, one group behind, hit the tee shot that led to an eagle and a two-stroke cushion.
"I played phenomenal golf and there's nothing I would change," Casey said after his best finish in a major.
"The glorious shots Collin hit like on 16 to make eagle, you have to tip your cap ... He's already sort of proved it but he's really stamped his authority of how good he is today."
Woods, who said he would skip next week's PGA Tour event, finished with a three-under 67 for a share of 37th place.
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