define(); and defined();?

I'm stuck trying to understand the the functions of the variables define(); and defined();. I'm making an admin page for my website and I have seen in other script they use something like this at the beginning of any admin page...

<?php defined( 'ADMIN_ACCESS' ) or die( '' ); ?> 

PHP:

So if someone is not an admin the script will die.
How can I do that? how and where do I define what ADMIN_ACCESS is? Thanks in advance!

 

defined() is identical to isset()

<?php defined( 'ADMIN_ACCESS' ) or die( '' ); ?> //<-- This line will check if the constant ADMIN_ACCESS is exists or not.

defined is only use to check the existing of constant.

define is use to create a new constant.

define('ADMIN_PASS','password123') //<-- That is and example of how you create a constant variable in PHP

 

define(); creates a constant. Use it as such:

define (CONSTANT, VALUE);

PHP:

defined(); checks if a constant has been... defined =)

define ('ADMIN_ACCESS', 'yes');

if (!defined('ADMIN_ACCESS') || !ADMIN_ACCESS) die('Not a valid entry point');

PHP:

Edit:
Oops.. php-lover submitted his reply just before mine =)

 

Thank you guys for your replies, OK now I see how that works, but how can I use that to block a regular member to access an admin page?

 

ahowell already give you the code:

define ('ADMIN_ACCESS', 'yes');

if (!defined('ADMIN_ACCESS') || !ADMIN_ACCESS) die('Not a valid entry point');

Code (markup):

But that is not so user friendly, you should redirect the user to the login page so the user could connect as admin if he/she can. die will stop the execution and nothing will be displayed in the page.

of course you should define the ADMIN_ACCESS with the value "yes" after the user is logged as admin.

 

Hey thanks for the reply. That's where I'm a little bit lost, where and how do I define the user is a admin? Can I use a cookie or a database? and If I can will this code..

if (!defined('ADMIN_ACCESS') || !ADMIN_ACCESS) die('Not a valid entry point');

PHP:

be able to tell who's an admin a who's not without me echoing out the cookie or the database?

Thanks for any help