You can define a constant by using the define()-function or by using the const keyword outside a class definition as of PHP 5.3.0. Once a constant is defined, it can never be changed or undefined.
Only scalar data (boolean, integer, float and string) can be contained in constants. It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results.
You can get the value of a constant by simply specifying its name. Unlike with variables, you should not prepend a constant with a $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name dynamically. Use get_defined_constants() to get a list of all defined constants.
Note: Constants and (global) variables are in a different namespace. This implies that for example TRUE and $TRUE are generally different.
If you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a string (CONSTANT vs "CONSTANT"). An error of level E_NOTICE will be issued when this happens. See also the manual entry on why $foo[bar] is wrong (unless you first define() bar as a constant). If you simply want to check if a constant is set, use the defined() function.
These are the differences between constants and variables:
Example #1 Defining Constants
<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.
?>
Example #2 Defining Constants using the const keyword
<?php
// Works as of PHP 5.3.0
const CONSTANT = 'Hello World';
echo CONSTANT;
?>
See also Class Constants.