Proper way to Include IPB core on external file

[Noni]

I have an external file in which in want to load the IPB core - init.php

I'm using this code in the index.php of a classifieds script (for cross login purpose)

/* Inlcude IPB core on login */
$path    = '/home/****/public_html/forum/';
require_once $path . 'init.php';

I'm getting a 500 server error which is not logged anywhere. 

In the external file, I've tried putting everything into a log file with error reporting set to 1, -1, on - the error does not appear in any log. 
error_reporting(-1); // reports all errors
ini_set("display_errors", "1"); // shows all errors
ini_set("log_errors", 1);
ini_set("error_log", "/home/****/public_html/errors.log");

Any suggestions on how to debug this would be appreciated...

My environment is: cloudlinux 6.x with php selector (yes the error reporting is enabled) and whm/cpanel.