After more than 50 years of friendship, Giancarlo Giammetti still knows how to keep a secret from fashion icon Valentino. Giammetti threw a “top-secret surprise birthday” for the jet-setting designer Thursday night in Miami, spies said. Guests at the intimate bash — at the Leading Hotels of the World’s The Setai — included Anne Hathaway (who arrived with her short ’do from the set of “Les Misérables,” and brought her mom as a Mother’s Day treat), Nacho Figueras, Countess Georgina Brandolini d’Adda, Carlos Souza and Leading Hotels ambassador Daniel Benedict. The swanky group surprised Valentino with a dinner that was unexpectedly interrupted by a (false) fire alarm and strobe lights. A spy said onlookers “at first thought the commotion might be for Valentino,” and that his fun-loving friends “applauded once the din stopped.” Hathaway, who also stars as Catwoman in “The Dark Knight Rises,” was seen during daylight hours hitting the beach in a sleek black bikini.