When you use PHP, you will likely have several includes files (with both PHP and HTML code) that make up a page. The include files may have only portions of the html for a given page within them, when combined by the server and sent to the viewer, they will act as one page.

Depending on how you set up your includes, you may or may not be able to work from within Design View for a portion of a given page.

To get around that possible issue, you can first create your page in html and hold onto that copy. As you piece it out into include files, you can always refer back to the original. When you make changes to the design, change it in the original first, then it's simple copy/paste to get the correct design code into the php includes.