PHP Warning: Unknown: failed to open stream

If you are running Fedora, make sure SELinux is not interfering.You fix this with this command:

    sudo /sbin/restorecon -R /var/www/

More info here: linuxquestions.org/questions/linux-server-73/

Experienced the same error, for me it was caused because on my Mac I have changed the DocumentRoot to my users Sites directory.

To fix it, I ran the recursive command to ensure that the Apache service has read permissions.

sudo chmod -R 755 ~/Sites