How to understand someone else's code of a php site?

Learn PHP from books (and not from someone else's code); when the knowledge base is large, then by itself learn to understand someone else's code.

Need to look for specific php scripts. If they have a call to a specific server (to where the file came from), then you need to replace these calls with the address of your server. If the script works "by itself", then you can simply upload it to your site's FTP server. If the script uses any resources of the server on which it is located (for example, it opens the image /images/image.png), then you need to find or create these resources and upload to the appropriate folder (that is, in this example, create in the directory with the php-script directory images and fill it with image.png), but again this is the script itself. If the script is associated with other scripts from this site, then they also need to be downloaded and uploaded to your server. And, of course, you need to view all the scripts and change specific data (for example, the name of the site, copyright, etc.).
Well, in general, in order to "figure out someone else's code" (that is, understand what and how this code does), you need to know the language of this code first, in this case PHP.

Usually included a display of errors, including, notice, and all corrected them. It is suitable for simple and medium-sized engines, as a rule, by itself. For complex (such as IPB, drupal) usually there is documentation.

Use advanced IDE, they will help you figure out where and where it comes from.

Look for exit points and from them carefully "spin" in the opposite direction. Pay attention to the design type
$$varname ....
$funcname() ....
compact() ....
list() ....
global ....
__get()/__set() ....
They can make life difficult.

Well, yes, you need knowledge of PHP, you can not take it away. =)