Sort MySQL query by a single match, then normally

I'm mildly familiar with MySQL queries, but I've got one tricky thing that I can't figure out how to do. Is there a way to tell the system "anything matching X-condition first, otherwise order normally." For example, in a table `posts` with the fields `(post_id, author_id, timestamp)` I know how to do... ``` mysql_query("SELECT * FROM posts ORDER BY timestamp"); ``` But is there a way to have a certain user's posts (say, `author_id = 5`) show up at the top of the results, and then sort by timestamp after that? To be clear: I DON'T want to sort by `author_id`, I just want to isolate the posts where `author_id` is 5 specifically, and then sort by `timestamp` for everything else. Is there a way to do that? I've tried, ``` mysql_query("SELECT * FROM posts ORDER BY author_id = 5, timestamp"); ``` ...but it doesn't seem to work.