PHILADELPHIA — The Islanders’ playoff hopes are all but gone now after a ghastly first period Thursday night.

Five Flyers goals zapped the lethargic Isles from the outset, a shocking display from a team that needed two points just to keep hope alive.

The 6-3 final, coupled with wins by the Maple Leafs and Bruins, has the Islanders six points out with six games to go and just about done for this season.

“What do you say? Nothing. It’s just [expletive],” an incensed Doug Weight said after his team lost a third straight in regulation for the second time this season. “They had four scoring chances in the first and it’s 5-0. Four! Do the math. And it’s absolutely floating, not engaged with anything.”

Thomas Greiss had to make a few good saves before the floodgates opened — and boy, did they open.

Dale Weise converted a two-on-one at 4:30. Sean Couturier banged home a rebound with Johnny Boychuk standing nearby at 5:49.

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When Radko Gudas’ slapper beat Greiss at 8:06, Weight summoned Jaroslav Halak to try to stop the bleeding. It didn’t work.

Jordan Weal jammed home his own rebound at 9:19 with three Islanders watching from behind the net and Wayne Simmonds had a point shot go off his leg, off Boychuk’s leg and in at 15:42 to leave the Islanders shell-shocked.

“We’re just making dumb mistakes that can’t happen and they’re in our net,” Casey Cizikas said. “There’s no words to describe that feeling.”

The Flyers scored almost as many goals in the opening 15:42 as they had in their previous three games combined against the Islanders this season (six). Nevertheless, they won the season series 3-1.

The Islanders tried to snap themselves out of it with a couple of first-period fights, but one of those was costly. Travis Hamonic had a soft cast on his left hand after leaving the game following a fight with Weise, likely meaning his season is done.

The Islanders managed to pull closer, though to no avail. Cal Clutterbuck, Jason Chimera and Andrew Ladd scored in the final two periods, but the Flyers didn’t need to do much of anything after their first 20 minutes. Valtteri Filppula scored into an empty net with 1:41 left to finish it off.

“Just disappointing, really. Embarrassing. All to really say about it,” John Tavares said. “We just were soft, just gave up way too much time and space. We’ve done it a few times before, given up goals quick like that . . . We’ve got six games left. Obviously, our situation is what it is. All we can control is how hard we work, how hard we play.”

They had those same things in their control going into the game, with a chance to at least keep pace in the playoff hunt. Now they aren’t even the first team on the edge of the race, with the Lightning three points ahead and the Hurricanes two ahead among teams chasing the final spot.

The Islanders simply looked unprepared for the moment.

“You cringe from the beginning of the year on,” said Weight, referring to the five games in the first 20 this season in which the Islanders lost in the final three minutes of regulation. “Not just those, but Detroit, St. Louis, the last few at home. You just cringe.”