Philip Johnson was a famous American architect, one of the foremost exponents of the International Style, responsible for many iconic structures. He died recently, but was active well into his nineties. He was also a sculptor, and worked in bronze, among other materials. While there probably are other Phil Johnsons that could have made this piece, if I owned it and wanted to find out more about it, I'd research the architect's art and see if there was any stylistic similarity. There's doubtless a gallery or two that handles his art; you might try getting in touch with them.