By default, and for security reasons, the only user that interacts with X-server is the logged-in user. Meaning, You can not initiate a connection to x-server as a different user or in your case as root, unless you have overridden the default setting to allow different users/hosts to connect to x-server. So the error message you are getting is related to x-server since any other user can't connect to it. In order to get around it, you have various fixes you can apply. What I normally do when ever I need to run an X-app as root is as follows.
- From normal user terminal, I run
xhost +to grant access to all user temporarily - Then I log in as root from a different terminal and execute the X-app ie
gedit tester.php
To make it permanent (which I don't suggest), edit /etc/profile and add the following line:
export XAUTHORITY=/root/.Xauthority