Syntax

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:

Constants do not have a dollar sign ($) before them;
Constants may only be defined using the define() function, not by simple assignment;
Constants may be defined and accessed anywhere without regard to variable scoping rules;
Constants may not be redefined or undefined once they have been set; and
Constants may only evaluate to scalar values.

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;
?>

Note:
As opposed to defining constants using define(), constants defined using the const keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops or if statements.