You can get this message if there is any white space outside of php script blocks on the page that is giving the error message. It is best to try and look at the source and remove any extra spaces or returns that are outside of the <?php ?> blocks.
This is also a problem that can happen if you select the option in Dreamweaver to Include Unicode Signature (BOM) On creating New pages or when saving, it is a PHP bug:
To fix the problem, Go to File -> Save As. In the Save As Dialogue box, uncheck the Include Unicode Signature (BOM) option.
You should make sure that this option is not set in Dreamweavers preferences as well.
Go to Edit -> Preferences, in the new Document section, uncheck the Include Unicode Signature (BOM) option.
Let us know how this goes for you.