OK, HERE'S a joke for you.

Why did Cory Booker cross the river?

No seriously, why did Booker - Democratic mayor of Newark, huge favorite to win the election in October as New Jersey's next U.S. senator, and friend to Oprah and Mark Zuckerberg - cross the river into Philadelphia last night?

Booker confessed to wondering the same thing after stealing the show at the 23rd annual Stu Bykofsky Candidates Comedy Night - but only after suffering through 90 minutes of the charity fundraiser, including the unique comic stylings of City Controller Alan Butkovitz.

"I've regretted some things in my life, but never have I regretted anything more than accepting this invitation tonight," joked Booker, confessing that he came at the urging of his "former friend" (heh) George Norcross, a South Jersey Democratic leader who's also Bykofsky's boss, as managing director of Daily News owner Interstate General Media.

Booker, Butkovitz, District Attorney Seth Williams and others traded their candidate caps for stand-up comedy mics at Finnigan's Wake in Northern Liberties for a good cause. The Daily News columnist has raised roughly $500,000 over the years for the Delaware Valley chapter of Variety, a charity that aids children with disabilities with medical equipment and other services.

That means that for one night, issues like Williams' briefly unpaid gas bill and Booker's penchant for high-profile deeds (he was accused at one point of "panda insemination" . . . don't ask) become the stuff of low comedy.

Self-confessed Yankees fan Booker conceded that he would pander to South Jersey voters. So he pretended to be a Philly sports fan, bragging, "I have played as many games for the Philadelphia 76ers as Andrew Bynum."

Meanwhile, Williams' Republican opponent (yes, he has one!) Danny Alvarez made the most of his brief, rare moment in the spotlight.

"I've been hitting the cemeteries because you all know that dead people vote in this town," Alvarez said. "In fact, I went to the cemetery and saw a Seth Williams bumper sticker on a tombstone."