The gang turns twenty.  

TV’s most outrageous sitcom “It’s Always Sunny In Philadelphia” is celebrating its 20th anniversary this summer – as the show first premiered on FX in the summer of 2005. 

Author Kimberly Potts’ new book out July 1, “It’s (Almost) Always Sunny in Philadelphia: How Three Friends Spent $200 to Create the Longest-Running Live-Action Sitcom in History and Help Build a Network,” details the show’s unlikely rise to prominence and behind the scenes stories. 

Potts, who has also written a book about “The Brady Bunch,” told The Post that during her research she learned how Danny DeVito nearly died while filming the series. 

During the Season 11 episode “The Gang Goes to Hell: Part Two,” the group is on a cruise, and are trapped in a room that has a leak. The episode aired on March 9, 2016.

“They’re swimming, they keep rising to the top. And to shoot that scene, they were underwater,” Potts explained, noting that the water level is rising.

“At one point, Danny got accidentally kicked, I think, in the shoulder –- close to his head. As I’ve been told, he nearly drowned,” she added. “It certainly had everyone afraid he was in trouble.” 

Created by Rob McElhenney and co-developed by Glenn Howerton, the show follows a group of narcissist and sociopathic friends who own a pub in the titular city: Dennis (Howerton), Charlie (Charlie Day), Mac (McElhenney), Dee (Kaitlin Olson), and Dennis and Dee’s father, Frank (Danny DeVito). 

After they rescued DeVito, “he was very frustrated by that situation,” Potts recalled to The Post.

“He just quietly left, and the day was over for him. So even he has a threshold for how far he’s willing to go. But for the overwhelming majority, their experience with him is great,” she acknowledged. “Kaitlin Olson has called him the happiest person she’s ever known.”

Potts’ book details the show’s unlikely success story, as the comedy’s original pilot cost Day, Howerton, and McElhenney a measly $200 to make. 

A few years later by 2009, Comedy Central would pay over $30 million to acquire syndication rights.

Potts attributed their success to “building slowly.”

“In the beginning, FX didn’t have a lot of money for marketing, so they’d do those wild creative marketing campaigns with graphics and go to college campuses. They had a huge college and high school fan base. Those people graduated, and now they have [teenage kids] they watch it with.” 

She also cited how YouTube launched the same year the show did, which helped certain scenes go viral. 

“With any story like this, certainly there is some luck involved,” she said.

“To do something like this now — in the current climate of TV — I think it would be almost impossible. People don’t get the chance to have that time and grow an audience. And gel as a cast and gel with writing staff and show people across several seasons what they can do.”

So, she said, many factors went into the mix of their unlikely success, including the fact that “they are legitimately friends in real life.” (McElhenney and Olson also met on the show and have been married since 2008.)

In 2009, “It’s Always Sunny” even brought a play on tour: “The Nightman Cometh,” which originated as a play within the show. The cast went to cities such as New York, Los Angeles, Philadelphia, and Seattle. 

“Danny DeVito rented a bus for them, because he thought they should feel like rock stars while doing a tour. He paid out of his own pocket for the tour bus,” Potts explained. 

She explained that this was before “intense social media.” 

“He had a bar installed, so they had a great time. They were all surprised when they would [visit these venues] and start to find out how much the show had grown.” 

That 2009 tour was a game changer for the show’s success, since they realized how many fans they had. 

As for when it could end? Howerton recently told The Post that they’ve discussed a conclusion — but revisit if they should continue “year by year.”

“We’re still having such a blast working with each other. And, there seems to be endless ways to explore the world through these characters. We have no intention of stopping anytime soon,” he revealed.

Potts cited cartoons that viewers have jokingly made of the cast doing the show into their old age, with canes and walkers. 

“I don’t think we’re gonna see ‘Sunny’ Season 50, probably not, anyway,” she reasoned. “But, can I see there being a Season 25? Maybe. I don’t think that’s out of the realm of possibility.”

Season 17 premieres on Wednesday, July 9 on FXX and Hulu.