The problem about the headers already being sent is probably one of the most common php errors that occur. Most of the time it is due to some white space outside of the php blocks, or having the BOM option enabled in DW for your pages. Have you looked for any white space outside of PHP blocks? Since the error is telling you that the output was already started at line 1 this tells me that you may have some type of white space occurring before the first line or on the first line on the page. Also, if you do a save as in DW check to make sure that you don't have the BOM option enabled.
Here is also a line of code you can put in your pages at the very top to see if it helps with this or gives you a different error message:
<?php if(!session_id()) session_start(); ?>
If you still have problems with this post back with the page in question and I can take a look for you to see if I can spot an issue.