set filename of file open/download

I'm using a php script to stream a microsoft word file to a browser. I'm sending header("Content-type: {$mimetype[DOC]} \n"); to set the mime type to application/msword so that the browser knows how to open the file. However, some users systems are configured to allow the choice of open or saveas. The problem is that when the user selects saveas the filename it uses is the name of the php script which streams the file, not the actual filename of the file. How do I set this filename?