What is the name of the movie where a kid writes on Stonehenge and him and his brother are taken to a futuristic sort of city?

The movie you're referring to is likely "The Kid Who Would Be King." In this film, a young boy named Alex discovers Excalibur and, along with his friends, must fight against an ancient evil. The story features elements of adventure and fantasy, including the exploration of a modernized world influenced by Arthurian legends.