array ( 0 => 'index.php', 1 => 'PHP Manual', ), 'head' => array ( 0 => 'UTF-8', 1 => 'es', ), 'this' => array ( 0 => 'language.constants.php', 1 => 'Constants', ), 'up' => array ( 0 => 'langref.php', 1 => 'Referencia del lenguaje', ), 'prev' => array ( 0 => 'language.variables.external.php', 1 => 'Variables desde fuentes externas', ), 'next' => array ( 0 => 'language.constants.syntax.php', 1 => 'Syntax', ), 'alternatives' => array ( ), 'source' => array ( 'lang' => 'en', 'path' => 'language/constants.xml', ), 'extra_header_links' => array ( 'rel' => 'alternate', 'href' => '/manual/en/feeds/language.constants.atom', 'type' => 'application/atom+xml', ), ); $setup["toc"] = $TOC; $setup["toc_deprecated"] = $TOC_DEPRECATED; $setup["parents"] = $PARENTS; manual_setup($setup); ?>
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 tambiĆ©n 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.