You can uses JavaScript's window.print() to make the browser print, doesn't even have to have user interaction, can print on page display or whenever you need. I am not sure what dialogs if any it might invoke on the operating system though, so there might be a confirmation dialog for print properties or something. You can also take a look at
http://us.php.net/manual/en/book.printer.php might need an additional extension to be added to your PHP install.