Chapter 8. Constants

Table of Contents
Predefined constants

A constant is a identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script. (The 'magic constants' __FILE__ and __LINE__ appear to be an exception to this, but they're not actually constants.) A constant is case-sensitive by default. By convention constant identifiers are always uppercase.

The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Note: For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 127 through 255 (0x7f-0xff).

The scope of a constant is global--you can access it anywhere in your script without regard to scope.


You can define a constant by using the define()-function. Once a constant is defined, it can never be changed or undefined.

Only scalar data (boolean, integer, double and string) can be contained in constants.

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 are 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. A notice will be issued when this happens. Use the defined()-function if you want to know if a constant is set.

These are the differences between constants and variables:

Example 8-1. Defining Constants

define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.