What's the best way to only allow a PHP file to be included?

$_SERVER['SCRIPT_FILENAME']

.htaccess

php.ini

php.ini

$parentPath = dirname(dirname(__FILE__));
$ourPath = $parentPath . DIRECTORY_SEPARATOR . 'include';

$includePath = ini_get('include_path');
$includePaths = explode(PATH_SEPARATOR, $includePath);
// Put our path between 'current directory' and rest of search path
if ($includePaths[0] == '.') {
    array_shift($includePaths);
}

array_unshift($includePaths, '.', $ourPath);
$includePath = implode(PATH_SEPARATOR, $includePaths);
ini_set('include_path', $includePath);

$parentPath = dirname(dirname(__FILE__));
$ourPath = $parentPath . DIRECTORY_SEPARATOR . 'include';

$includePath = ini_get('include_path');
$includePaths = explode(PATH_SEPARATOR, $includePath);
// Put our path between 'current directory' and rest of search path
if ($includePaths[0] == '.') {
    array_shift($includePaths);
}

array_unshift($includePaths, '.', $ourPath);
$includePath = implode(PATH_SEPARATOR, $includePaths);
ini_set('include_path', $includePath);

<?php
if (!eregi("modules.php", $PHP_SELF)) {
   die ("You can't access this file directly...");
}
// more code ...
?>

if(!isset($in_prog)){
exit;
}

// index.php
require_once 'includes.php';

// includes.php
define('IN_MY_PROJECT', true);
include 'myInc.php';

// myInc.php
defined('IN_MY_PROJECT') || die("No direct access, plsktnxbai");