Constants (original) (raw)

Table of Contents

• Syntax
• Magic constants

A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for magic constants, which aren't 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 thusly:^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$

It is possible to define() constants with reserved or even invalid names, whose value can (only) be retrieved withconstant(). However, doing so is not recommended.

Example #1 Valid and invalid constant names

<?php// Valid constant names define("FOO", "something"); define("FOO2", "something else"); define("FOO_BAR", "something more");// Invalid constant names define("2FOO", "something");// This is valid, but should be avoided: // PHP may one day provide a magical constant // that will break your script define("__FOO__", "something"); ?>

Note: For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 128 through 255 (0x80-0xff).

Like superglobals, the scope of a constant is global. You can access constants anywhere in your script without regard to scope. For more information on scope, read the manual section onvariable scope.