Dreamweaver will use the virtual() function for attaching php pages when the Dreamweaver site is configured to "Create Links Relative to:" "Site". if you change this setting to "Document", Dreamweaver will use the require_once() function instead.
The virtual() function is only available in some Apache server environments.
Go to Site -> Manage Sites. in the site manager, select your site and click the edit button. On the local info tab, change the "Create Links Relative to:" option to "Document"
after making this change, you will need to edit or recreate the site assist site for it to regenerate the code.