   #PHP Manual Language Reference Variables from outside PHP Predefined
   constants

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Chapter 8. Constants

   Table of Contents
   Syntax
   Predefined 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_\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).

   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 on variable 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
   wish 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, just as if you called it as a string (CONSTANT
   vs "CONSTANT"). An error of level E_NOTICE will be issued when this
   happens. See also the manual entry on why $foo[bar] is wrong (unless
   you first define() bar as a constant). If you simply want to check if
   a constant is set, use the defined() function.

   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 8-1. Defining Constants
   <?php
   define("CONSTANT", "Hello world.");
   echo CONSTANT; // outputs "Hello world."
   echo Constant; // outputs "Constant" and issues a notice.
   ?>
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