Best Practices for calling 'setup' classes that extend 'parent' classes?

What is the best practice for correctly targeting 'include' paths when using Initialization/Setup classes that extend Parent classes? In my extend_parent_class.php file, I have to provide an incorrect relative path. Apparently, the path (that does work) is relative to php file that calls the initialization/setup class (call_the_extend_parent_class.php). Is there a less confusing way to correctly target include paths when creating initialization classes? Should I be using absolute paths instead? many thanks in advance extend_parent_class.php: include_once('./path/to/parent_class.php'); # Works with 'incorrect' relative path //include_once('../path/to/parent_class.php'); # Fatal Error with 'correct' relative path class Initialize_Parent_Class extends Parent_Class { function Initialize_Parent_Class() { $this->Parent_Class(); echo "This was successful and does not result in a fatal 'class not found' error"; } } Call the initialization class. call_the_extend_parent_class.php: <?php require('./includes/extend_parent_class.php'); # initialize Parent Class $parent_class = new Initialize_Parent_Class(); // prints 'This was successful and does not result in a fatal 'class not found' error' ?> File structure: php ++call_the_extend_parent_class.php ++ includes directory ++++ extend_parent_class.php ++ classes directory ++++ parent_class.php