What was the real name of the great American Short story writer, O' Henry?

O' Henry was an American famous short story writer. His original name was William Sidney Porter. O' Henry is his Penname. He is famous for his story The Gift of the Magi. Samuel Langhorne Clemens is the real name of Mark Twain and Mary Ann Evans is the real name of George Eliot.
Source: Encyclopedia of Britannica.