ScotlandPHP

Constants

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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]*

Tip

See also the Userland Naming Guide.

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 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.

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User Contributed Notes 11 notes

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157
wbcarts at juno dot com
5 years ago
11/14/2016 - note updated by sobak
-----

CONSTANTS and PHP Class Definitions

Using "define('MY_VAR', 'default value')" INSIDE a class definition does not work as expected. You have to use the PHP keyword 'const' and initialize it with a scalar value -- boolean, int, float, string (or array in PHP 5.6+) -- right away.

<?php

define
('MIN_VALUE', '0.0');   // RIGHT - Works OUTSIDE of a class definition.
define('MAX_VALUE', '1.0');   // RIGHT - Works OUTSIDE of a class definition.

//const MIN_VALUE = 0.0;         RIGHT - Works both INSIDE and OUTSIDE of a class definition.
//const MAX_VALUE = 1.0;         RIGHT - Works both INSIDE and OUTSIDE of a class definition.

class Constants
{
 
//define('MIN_VALUE', '0.0');  WRONG - Works OUTSIDE of a class definition.
  //define('MAX_VALUE', '1.0');  WRONG - Works OUTSIDE of a class definition.

 
const MIN_VALUE = 0.0;      // RIGHT - Works INSIDE of a class definition.
 
const MAX_VALUE = 1.0;      // RIGHT - Works INSIDE of a class definition.

 
public static function getMinValue()
  {
    return
self::MIN_VALUE;
  }

  public static function
getMaxValue()
  {
    return
self::MAX_VALUE;
  }
}

?>

#Example 1:
You can access these constants DIRECTLY like so:
* type the class name exactly.
* type two (2) colons.
* type the const name exactly.

#Example 2:
Because our class definition provides two (2) static functions, you can also access them like so:
* type the class name exactly.
* type two (2) colons.
* type the function name exactly (with the parentheses).

<?php

#Example 1:
$min = Constants::MIN_VALUE;
$max = Constants::MAX_VALUE;

#Example 2:
$min = Constants::getMinValue();
$max = Constants::getMaxValue();

?>

Once class constants are declared AND initialized, they cannot be set to different values -- that is why there are no setMinValue() and setMaxValue() functions in the class definition -- which means they are READ-ONLY and STATIC (shared by all instances of the class).
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10
Raheel Khan
2 years ago
class constant are by default public in nature but they cannot be assigned visibility factor and in turn gives syntax error

<?php

class constants {

    const
MAX_VALUE = 10;
        public const
MIN_VALUE =1;

}

// This will work
echo constants::MAX_VALUE;

// This will return syntax error
echo constants::MIN_VALUE;
?>
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15
katana at katana-inc dot com
15 years ago
Warning, constants used within the heredoc syntax (http://www.php.net/manual/en/language.types.string.php) are not interpreted!

Editor's Note: This is true. PHP has no way of recognizing the constant from any other string of characters within the heredoc block.
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6
gried at NOSPAM dot nsys dot by
1 year ago
Lets expand comment of 'storm' about usage of undefined constants. His claim that 'An undefined constant evaluates as true...' is wrong and right at same time. As said further in documentation ' 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...'. So yeah, undefined global constant when accessed directly will be resolved as string equal to name of sought constant (as thought PHP supposes that programmer had forgot apostrophes and autofixes it) and non-zero non-empty string converts to True.

There are two ways to prevent this:
1. always use function constant('CONST_NAME') to get constant value (BTW it also works for class constants - constant('CLASS_NAME::CONST_NAME') );
2. use only class constants (that are defined inside of class using keyword const) because they are not converted to string when not found but throw exception instead (Fatal error: Undefined class constant).
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20
storm
12 years ago
An undefined constant evaluates as true when not used correctly. Say for example you had something like this:

settings.php
<?php
// Debug mode
define('DEBUG',false);
?>

test.php
<?php
include('settings.php');

if (
DEBUG) {
  
// echo some sensitive data.
}
?>

If for some reason settings.php doesn't get included and the DEBUG constant is not set, PHP will STILL print the sensitive data. The solution is to evaluate it. Like so:

settings.php
<?php
// Debug mode
define('DEBUG',0);
?>

test.php
<?php
include('settings.php');

if (
DEBUG == 1) {
  
// echo some sensitive data.
}
?>

Now it works correctly.
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12
ewspencer at industrex dot com
14 years ago
I find using the concatenation operator helps disambiguate value assignments with constants. For example, setting constants in a global configuration file:

<?php
define
('LOCATOR',   "/locator");
define('CLASSES',   LOCATOR."/code/classes");
define('FUNCTIONS', LOCATOR."/code/functions");
define('USERDIR',   LOCATOR."/user");
?>

Later, I can use the same convention when invoking a constant's value for static constructs such as require() calls:

<?php
require_once(FUNCTIONS."/database.fnc");
require_once(
FUNCTIONS."/randchar.fnc");
?>

as well as dynamic constructs, typical of value assignment to variables:

<?php
$userid 
= randchar(8,'anc','u');
$usermap = USERDIR."/".$userid.".png";
?>

The above convention works for me, and helps produce self-documenting code.

-- Erich
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6
Andreas R.
10 years ago
If you are looking for predefined constants like
* PHP_OS (to show the operating system, PHP was compiled for; php_uname('s') might be more suitable),
* DIRECTORY_SEPARATOR ("\\" on Win, '/' Linux,...)
* PATH_SEPARATOR (';' on Win, ':' on Linux,...)
they are buried in 'Predefined Constants' under 'List of Reserved Words' in the appendix:
http://www.php.net/manual/en/reserved.constants.php
while the latter two are also mentioned in 'Directory Functions'
http://www.php.net/manual/en/ref.dir.php
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5
hafenator2000 at yahoo dot com
12 years ago
PHP Modules also define constants.  Make sure to avoid constant name collisions.  There are two ways to do this that I can think of.
First: in your code make sure that the constant name is not already used.  ex. <?php if (! defined("CONSTANT_NAME")) { Define("CONSTANT_NAME","Some Value"); } ?>  This can get messy when you start thinking about collision handling, and the implications of this.
Second: Use some off prepend to all your constant names without exception  ex. <?php Define("SITE_CONSTANT_NAME","Some Value"); ?>

Perhaps the developers or documentation maintainers could recommend a good prepend and ask module writers to avoid that prepend in modules.
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1
jcastromail at yahoo dot es
14 days ago
Performance of constants.  PHP 7.1.10 32 bits (Opcache active, windows 10 i7-64bits) but apparently the trends is the same with the 5.x

using a constant declared by DEFINE('CNS',value) : 0.63575601577759s
using a constant declared by const CNS=value : 0.61372208595276s
using a variable declared by $v=value : 0.51184010505676s

In average, the use of DEFINE and CONST is around the same with some sightly  better performance of CONST instead of DEFINE. However, using a variable is around 10-50% better than to use a constant.  So, for a performance intensive task, constant is not the best option.

$p1=microtime(true);
$x=0;
for($i=0;$i<50000000;$i++) {
    $x+=CNS;
}
$p2=microtime(true);
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-8
xs2ilyas at gmail dot com
8 months ago
When we start a constant name with space, it doesn't produce any error.

<?php
  define
(" YEAR"2000)
  echo
"success";
?>

output: success

But when we call this constant, it produce error.

<?php
  define
(" YEAR"2000)
  echo
YEAR;
?>

output: E_NOTICE :  type 8 -- Use of undefined constant YEAR - assumed 'YEAR' -- at line 4 YEAR
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-18
php at webflips dot net
3 years ago
It is perfectly valid to use a built-in PHP keyword as a constant name - as long as you the constant() function to retrieve it later:

<?php
define
('echo', 'My constant value');

echo
constant('echo'); // outputs 'My constant value'
?>
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