PHP/Ubuntu - QxcbConnection: Could not connect to display aborted
Okay, I finally managed to solve it.
The issue is not with the permissions, but it is with the environment variables.
I had to include the following line in my bash script
export DISPLAY=':0.0'
Note that setting the variable in the terminal and running the script does not work. The line needs to be inside the script.
I assume this is because the DISPLAY variable is not set if you run the script as any user other than root, which is what happens in case of Apache/PHP where the script is executed as the 'www-data' user.