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# Chapter 9. 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.

# Syntax

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, float 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:

- 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 9-1. Defining Constants
    <?php
    define("CONSTANT", "Hello world.");
    echo CONSTANT; // outputs "Hello world."
    echo Constant; // outputs "Constant" and issues a notice.
    ?>

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