GLENDALE, Ariz. — The Giants could make a fortune on the black market selling whatever unknown magic elixir was passed around the halftime locker room.

The worst offense in the NFL to that point — held scoreless for the first 91-plus minutes of the season — became unstoppable and a fundamentally flawed defense became impenetrable, and the combination of the two turnarounds Sunday saved the Giants’ season from a shockingly early point of no return.

Five straight scoring drives, four straight defensive stops and the closed-doors urgency expressed by head coach Brian Daboll at State Farm Stadium added up to a 31-28 win against the hapless Cardinals, whose 20-point halftime lead was a continuation of the avalanche pouring down on the Giants since their 40-point loss to the Cowboys in Week 1.

“Nobody was flipping tables, but it wasn’t too soft,” receiver Darius Slayton told The Post about halftime. “People voiced their opinions. In light of last week it was, ‘We’re not about to let this be, here we go again.’ Everybody understood that there’s only one way we get ourselves out of this hole: Players making plays.”

Saquon Barkley scored one touchdown on the ground and another through the air, complete with a full-extension dive to the pylon, 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.

The Giants’ 21-point comeback (trailing 28-7) was their largest since 1949, according to Elias Sports Bureau.

“It’s not always easy to be mentally tough when you’re [down] 60-0, after a game and a half,” Daboll said. “There’s encouragement. There is detail in what needs to get done. Scream and yell, I don’t think that’s the answer. But you definitely place an emphasis on what needs to be done, how it needs to be done, with confidence and sternness.”

The Giants still have big concerns because Barkley is headed for more testing on a sprained right ankle, but are not facing the biggest question of all — what to do with 15 meaningless games? — as would have been the case after an 0-2 start with a loss to arguably the least-talented roster in the league.

Jones threw for 321 yards — maybe none bigger than the 58 that came on a second-half-opening deep post to Jalin Hyatt drawn up during that seminal halftime locker room.

“It wasn’t a riot act,” safety Xavier McKinney told The Post. “You have to execute. You have to play fast. You have to make plays. We didn’t have a whole bunch of rah-rah speeches. At a point, there’s only so much talking anyone is going to do.”

Even once the offense got started, there was no reason to think 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 the Cardinals capitalized with a touchdown to go up 28-7.

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 easy completions for journeyman quarterback Josh Dobbs dried up.

“One thing we did say going out for the second half was, ‘No more points on defense,’ ” defensive tackle Leonard Williams said. “The thing is we’ve done it before. This is a different team — we don’t like to go back to last year under any circumstances — but we showed we could do that last year. One of our mottos is, ‘Take them to the deep end,’ and we did a good job. Once the offense started scoring and the defense started getting stops, we started to get a lot more belief.”

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 Dobbs running through McKinney at the goal line?

Or Jalen Thompson’s interception off of Barkley’s hands? Or Marquise Brown and Ertz running open all throughout the secondary?

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 Daniel Jones was sacked against a three-man pass rush during their five first-half possessions?

None of it sat well with Daboll.

“He’s that fiery, passionate same message he always gives,” Jones said, “and guys took to it.”