When you get the headers already sent message like this it is usually because of some white space or other characters that occur outside of the php blocks. This can normally be corrected by examining the tags on the page near the error so that you find and remove any white space.
When you get the error like this at line one it can sometimes be due to a special character that is encoded and set as the first character of the page. In DW you can test for this by doing a save as for the page, in the dialog that appears make sure that you do not have the option for 'Include Unicode Signature (BOM)' checked. Having this option enabled can put this extra character at the beginning of your file and cause this error.
Please try these things and post back with what you discover.