This is a Dreamweaver issue.
When defining your site, There is an Option is the Site definition settings:
"Create Links Relative To"
using the Site option causes DW to use the virtual() function for included files. This function is only defined on some PH server, and even when it is defined, it writes a bit to the page header that causes the header already sent error on redirects.
instead use the Document settings so that DW will use the require_once() function for including php files. the require_once() function does not include the issues that virtual() does.