Did you leave a space in the code or something? often a space or empty line will cause this error on a php page. Look at the checkout.php page on line 2 and see if there is a blank line or space character in the code.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.