Thought I'd write a small tutorial, so here goes.

In php you can have variables, which you can in pretty much every language, and you can have constants, a form of variable with it's own properties, which I'll list now:

• A constant once defined, cannot be changed or undefined
  
• A constant must start with a letter (a-z) or an underscore, it may contain numbers too though, just not at the beginning.
  
• Constants may only contain scalar data (boolean, int, string and float)
  
• A constant doesn't comply with the variable-scope.
  

The last point there is pretty much why constants are good (and why they are called constants really). You can access the value of a constant (or check if it's defined) -anywhere- in your software, it could be a function, an oop wrapper, a function inside an oop wrapper, anywhere.

So, how do we define constants? It's really easy, take a look:

define('CONS_NAME', 'CONS_VALUE');

How can you get the value? Simply by calling it's name, for example:
echo CONS_NAME;

Or you can use constant();, more info on that here: uk.php.net/manual/en/function.constant.php

So how about checking if the constant exists?
if (defined('CONS_NAME'))
{
echo 'constant is defined';
}

That's all easy then, what about a good scenario for a constant, well, let's say your software all ran through 1 file, index.php for instance, index.php however included/required other files, say functions.php and such, all good, but what if you didn't want users to access functions.php via there browser? Simple.

In index.php you'd place:
define('IN_SYS', true);

At the top (below <?php/<?) of functions.php
if(!defined('IN_SYS'))
{
die('<h4>You may not access this script directly.</h4>');
}

Well, I'm sure you can put it to use as you see fit, hope this helped

- Zero Tolerance