I've been working on not-much-complicated projects using CodeIgniter on windows, but this is my first time to launch any php file in Ubuntu.
I copied a CodeIgniter project to
sudo chmod 777 -R /opt/lampp/htdocs
sudo chmod 777 -R /opt/lampp/htdocs/ PROJECT_FOLDER
In addition to the good comments, apache needs execute access on every folder from root to the project in order to traverse the file system.
sudo chmod +x / sudo chmod +x /opt sudo chmod +x /opt/lampp