If it is just a headers issue you should be able to resolve this by finding some white space around the php blocks and deleting it, also there is a BOM option you can have when you save pages in DW, if you have this option enabled for the page you should try disabling it when you save the page. Here is a link to an article that has more info on the white spaces:
If this does not clear up the problem then the issue may be with the session starting and not be able to write the session cookies. If this is the case there may be an issue with your session save path info for you PHP. If you have access to this you should check the php.ini file for this setting and make sure it is set correctly. If you do not have access to this or do not know how to update it you should speak with your host about this.