require_once and direct script access security

[Samishii23]

I read up that sometimes require_once() still will reload a script file or w/e sometimes. Should I do something like this to prevent overhead?

// Script file
if ( isset(FILE_LIB) ) exit; // Prevent double loading

define('FILE_LIB', 1);
...
...

Is this reasonable or no?

Also, to stop people from accessing ( not using .htaccess ) other scripts then desired, like the above mentioned library file...

// index.php
define('MY_SCRIPT', 1);

// library.php
if ( !isset(MY_SCRIPT) ) exit;

Or should I redirect the user to index.php???

[Markus]

Where did you read that?

Also, you should be using defined(), not isset(), to check where a constant is defined.

[Samishii23]

One of the random articles I've been coming accross while researching Good PHP Practices, and PHP security Tricks.