I added a form to a page in Muse (via Object > Insert HTML) and I moved the php file and associated folder from the _exported folder to the Muse Export folder. When I do a page or site preview, I always get an error message that the .php file cannot be found.

Here is the generated HTML:

<script type="text/javascript">document.write(unescape("%3Ciframe id=\"fb_iframe\" src=\"testform3.php" + window.location.search + "\" width=\"600\" height=\"755\"allowtransparency=\"true\" scrolling=\"no\" frameborder=\"0\"%3E&lt;a href=\"testform3.php\" title=\"testform3\"&gt;Check out my CoffeeCup Form&lt;/a&gt;%3C/iframe%3E"));</script>
<noscript>
<iframe width="600" height="755" style="border:none; background:transparent; overflow:hidden;"
id="fb_iframe" src="testform3/testform3.html">
<a href="testform3.php" title="testform3">Check out my CoffeeCup Form</a>
</iframe>
</noscript>

Make sure that the inner folder and the .php file that are inside the _exported folder are located within your site's root directory or within the directory that the page the form code is on is residing. That's usually where the issue comes in. You cannot put these files within another folder of your site without going in and changing the locations in the code.