Could it be magic at the Broadway box office, or could it be Manilow?Turns out Barry Manilow can draw big crowds on the Main Stem — which makes sense given the overlap between the performer’s fanbase and the primarily older-skewing demo of Broadway theatergoers. Last week the first two performances of Manilow’s Rialto concert gig, “Manilow on Broadway,” pulled in a whopping $447,956, putting the show’s per-perf numbers on par with those of Main Stem juggernauts like “Wicked” ($1,700,024) and “The Book of Mormon” ($1,655,074) and making it clear just why producers opted to extend the show’s limited run by two weeks, now closing Feb. 23. Meanwhile, one of the biggest bumps of the frame, both in percentage terms and concrete dollar figures, was at “The Heiress” ($596,439), up $150,000 or 34%. That’s particularly notable because the revival stars Jessica Chastain — who the same weekend pulled in a major haul at the multiplex with both “Mama” and “Zero Dark Thirty.” Overall Broadway cume rose, given a tourism boost by the long weekend enabled by the Martin Luther King Jr. Day holiday. There was also some last-minute biz at the three shows that closed over the weekend: “Glengarry Glen Ross” ($1,070,538), the Al Pacino starrer that returned to the millionaire’s club in its final sesh on the boards, “Peter and the Starcatcher” ($502,223) and “Golden Boy” ($471,181). “Evita” ($1,108,693) also benefitted from a combination of long-weekend traffic and final-days enthusiasm, with the Ricky Martin starrer set to close this week. “Cat on a Hot Tin Roof” ($891,457) rose a bit despite not only accommodating some press perfs and a comp-heavy opening night but also weathering some unenthusiastic notices among its mixed bag of reviews. The Scarlett Johansson topliner looks poised to snatch the crown for the Rialto’s top-selling play now that “Glengarry” has taken its final bow. Overall Broadway total stepped up by $1.7 million to $19.6 million for 25 shows on the boards, with attendance up by about 10,000 to 196,399. But don’t look for the momentum to last: With three shows just closed and one more on its way out — and no holiday weekend to offer a lift — B.O. looks poised to return to the usual January doldrums, with only “Manilow” to take up the slack for now.