A function to check if the call was internal, since PHP does not have the internal access modifier.

Internal.php

<?php

abstract class Internal {

    protected function checkInternalCall() {
        $bt = debug_backtrace();
        if (count($bt) < 2 || !isset($bt[1]['class']) || substr($bt[1]['class'], 0, strlen(__NAMESPACE__) + 1) != __NAMESPACE__ . "\\")
            throw new \Exception("This function is marked as 'internal' and cannot be called outside of the namespace");
    }

}