GLENDALE, Ariz. — Coaches everywhere would drain their bank accounts on the black market to buy a bottle of whatever magic happened Sunday in the Giants’ halftime locker room.

The worst offense in the NFL — held scoreless for the first 91:17 of the season — became unstoppable as it scored on all five of its second-half possessions and saved the Giants’ season from a shockingly early point of no return with a 31-28 victory over the hapless Cardinals at State Farm Stadium.

As the Giants emerged from halftime trailing 20-0, it was a question of whether they could get first downs, let alone score. Forget about winning.

Whether head coach Brian Daboll read the riot act or used a softer touch, Saquon Barkley scored two touchdowns, Daniel Jones rushed for one and threw another to Isaiah Hodgins, and Graham Gano kicked the tie-breaking 34-yard field goal with 19 seconds remaining to complete the improbable comeback. Jones threw for 321 yards, with Darren Waller’s six catches for 76 yards sparking the offense.

The Giants still have big concerns because the injury-plagued Barkley was carried off the field with attention paid to his ankle after fighting for yardage on the winning drive, but not the biggest question of all: What to do with 15 meaningless games?

Like a frustrated Madden video-game user, play-caller Mike Kafka came out of halftime throwing a bomb to the fastest guy out of the locker room. It worked for a 58-yard completion to Jalin Hyatt — a make-up for Jones underthrowing a potential long touchdown to Darius Slayton in the first half.

Jones walked-in untouched on a 14-yard read-option touchdown, and the Giants’ offense never lost its spark. Barkley scored the next two touchdowns — one rushing and one receiving complete with an incredible stretch across the goal line — to cut the deficit to 28-21 with 8:51 to go.

There still was no reason to think that the Giants had the defense to make a comeback. Not after the Cardinals scored three touchdowns and two field goals on their first six possessions, and the only “stop” was aided by an overthrown would-be touchdown and a missed field goal. Not after an interception by Jason Pinnock was negated because Bobby Okereke wrestled Zach Ertz to the ground for pass interference.

And then things changed. The ankle grabs and arm reaches that James Conner was running through turned into tackles and those tackles led to three straight punts.

The Giants were supposed to see the NFL’s worst team Sunday in Arizona. For a half, they might have just by looking in a mirror.

If the Cardinals were tanking for the No. 1 pick in the 2024 draft — as has been widely accused — how do you explain quarterback Josh Dobbs running through Xavier McKinney at the goal line? Or Jalen Thompson’s interception off of Barkley’s hands? Or Marquise Brown and Zach Ertz running open all throughout the secondary, Conner breaking tackles and the pressure on an overmatched offensive line?

Or any of the miscues that created a lopsided score as the Giants punted three times, committed a turnover and mercifully let the clock run out when Jones was sacked against a three-man pass rush during their five first-half possessions.

As bad as the season-opening 40-0 demolition by the Cowboys was, there were excuses like … the Cowboys are one of the NFL’s best teams … or it’s a bad matchup on the lines of scrimmage for the Giants … or fluky things happen in Week 1.

There would have been no masking the stench of losing to the Cardinals, whose plan without $230 million franchise quarterback Kyler Murray was to cut projected starter Colt McCoy after training camp, trade for Dobbs and hand the reins to the journeyman backup.

The halftime box score showed the Cardinals with edges of 16-5 in first downs and 241-81 in yards. It was the worst first half imaginable … well, expect for allowing touchdowns in all three phases and falling behind 26-0 to the Cowboys last week.

Barkley curiously didn’t get a carry on two of the first three drives, both of which went three-and-out. When the Giants finally tried to get their best playmaker involved, it was disastrous. For the second straight game, a simple checkdown pass to Barkley wound up popping into the air for an interception.

But the final pass floating in the air was a Hail Mary from Dobbs that landed at the 10-yard line and sent the Giants’ sideline spilling onto the field in the unlikeliest of celebrations.