Constants
While most constants are only defined in one namespace, the case-insensitive true, false, and null constants are defined in ALL namespaces. So, this is not valid:
<?php namespace false;
const ENT_QUOTES = 'My value';
echo ENT_QUOTES;//Outputs as expected: 'My value'
const FALSE = 'Odd, eh?';//FATAL ERROR! ?>
Fatal error: Cannot redeclare constant 'FALSE' in /Volumes/WebServer/0gb.us/test.php on line 5
Syntax
You can define a constant by using the define()-function or by using the const keyword outside a class definition as of PHP 5.3.0. 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. It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results.
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
TRUEand $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 #1 Defining Constants
<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.
?>
Example #2 Defining Constants using the const keyword
<?php
// Works as of PHP 5.3.0
const CONSTANT = 'Hello World';
echo CONSTANT;
?>
Note:
As opposed to defining constants using define(), constants defined using the const keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops or if statements.
See also Class Constants.
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User Contributed Notes
Syntax - [3 notes]
0gb dot us at 0gb dot us ¶
1 year ago
uramihsayibok, gmail, com ¶
3 years ago
Don't let the comparison between const (in the global context) and define() confuse you: while define() allows expressions as the value, const does not. In that sense it behaves exactly as const (in class context) does.
<?php
// this works
/**
* Path to the root of the application
*/
define("PATH_ROOT", dirname(__FILE__));
// this does not
/**
* Path to configuration files
*/
const PATH_CONFIG = PATH_ROOT . "/config";
// this does
/**
* Path to configuration files - DEPRECATED, use PATH_CONFIG
*/
const PATH_CONF = PATH_CONFIG;
?>
timucinbahsi at gmail dot com ¶
1 year ago
Constant names shouldn't include operators. Otherwise php doesn't take them as part of the constant name and tries to evaluate them:
<?php
define("SALARY-WORK",0.02); // set the proportion
$salary=SALARY-WORK*$work; // tries to subtract WORK times $work from SALARY
?>