delete user but keep the article

hi, i have a news article database that input by users, for example articlename: 'abc', input by username: 'user_x'. i want to delete user_x, but keep article abc. how php/mysql display that there is article named 'abc', input by 'user_x', but there are no further information about that user (except maybe only his firstname) because user_x no longer exists. how can i do this? thanks rudy.