If you're unsure of how to reach the PHP Settings section in Plesk, please see this guide.
This guide will assume that you have a domain (example.com) and you need it to access files on a different domain that is also on the same server (example.org).
For reference, the web root of all websites on your server are in this format:
Now that we know the basics of where things are, we can make the changes.
Go into the PHP Settings page for the domain that needs access to the second domain (example.com in this case, use the guide at the top of this article if you're not sure how to find this area in Plesk).
Then change the open_basedir option to Enter custom value and append your domain's full path to the entries in the box. You should end up with something that looks like this :
What this means is that the domain you are currently modifying (example.com) can now access its own files (webspaceroot), the server's temp folder (tmp) and the other domain you have added (/var/www/vhosts/example.org/httpdocs).
You can add as many locations as you need, separated by colons. Once you've done that, you should get something that looks like this:
Click Ok at the bottom of the page to save your changes.