Adding a php page to the dolphin menu is actually quite simple.
1. Utilizing your FTP Client set to "Binary" transfers, upload the files to your Dolphin Root. This would be the directory that has things like upload_media.php, faq.php, member.php & so on in it. The first folder in this directory is "admin".
2. Go to yoursite.com/admin & login. Then go to Builders -> Navigation Menu Builder and drag a new item block to the top line or whatever column you would like to make it a submenu item of. Basically, just place it where you want it to appear in the menu.
3. Open the New Item block up by clicking on it. Give it a name (this is for the administration of it) next give it a title (this will appear as the menu name on the endusers screen)
4. Finally, for the url you simply need to put the file name item.php is an example. No need to put http://yoursite.com/item.php (actually if you do use the http then it will cause errors in the menu)
Save it and your done. If the page needs mod_rewrite then you need to add the appropriate content to your sites .htaccess file.
Good luck with your project.