Q:

Failed to open stream even though place exists

I'm trying to include a file using:
require_once '../config/configVARS.php';

but I get the error:
Warning: require_once(../config/configVARS.php): failed to open stream: No such file or directory in ..../classes/FileHandler.php on line 2

Fatal error: require_once(): Failed opening required '../config/configVARS.php' (include_path='.:/opt/lampp/lib/php') in ..../classes/FileHandler.php on line 2

If I try to include a file on the same folder i don't have any error (for ex. when I copy the configVARS file to the same folder i'm able to include it).
I checked permissions and that's not it (neither ownership).
Any ideas? what can cause it?

A:

Check that the file actually is there, if you are unsure that you are using the right file-name, use an absolute path instead of a relative one.
The magic constant __DIR__Docs contains the absolute location of the file you write it into, so 
require_once __DIR__ . '/concrete/path/to/file.php';

is often more distinct as you actually seem to know where exactly the file exists - a pre-configured include directory where to look in does not seem necessary.

