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). Constants are case-sensitive. By convention, constant identifiers are always uppercase.
Nota:
Prior to PHP 8.0.0, constants defined using the define() function may be case-insensitive.
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 with the constant() function. However, doing so is not recommended.
Vea también Guía de entorno de usuario para nombres.
Ejemplo #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");
?>Nota: 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. Constants can be accessed from anywhere in a script without regard to scope. For more information on scope, read the manual section on variable scope.
Nota: As of PHP 7.1.0, class constant may declare a visibility of protected or private, making them only available in the hierarchical scope of the class in which it is defined.