ScotlandPHP 2016

Costanti

Indice dei contenuti

Una costante è un identificatore (nome) per un valore. Come si può intuire, tale valore non può cambiare durante l'esecuzione dello script (fanno eccezione le costanti magiche, che, in realtà, non sono costanti). Una costante è "case-sensitive" per default. È convenzione comune che i nomi di costante siano sempre maiuscoli.

In PHP il nome di una costante segue le regole di qualsiasi "etichetta". Un nome di costante valido inizia con una lettera o underscore, seguita da un numero qualsiasi di caratteri alfanumerici o underscore. L'espressione regolare che esprime questa convenzione è: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Suggerimento

Si può anche consultare Guida Denominazione Spazio Utente.

Example #1 Nomi di costanti validi ed errati

<?php

// Nomi validi
define("FOO",     "something");
define("FOO2",    "something else");
define("FOO_BAR""something more")

// Nomi di costante errati
define("2FOO",    "something");

// Nomi validi, ma da evitare:
// un domani potrebbero essere utilizzati dal PHP per fornire costanti magiche
// e quindi si avrebbero problemi nello script
define("__FOO__""something");

?>

Nota: In questo contesto una lettera è a-z, A-Z e i caratteri ASCII dal 127 al 255 (0x7f-0xff).

Come le superglobals, costante è sempre globale. Si può accedere alle costanti da qualsiasi punto dello script senza tenere conto della visibilità. Per maggiori dettagli sulla visibilità, leggere la sezione variable scope.

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User Contributed Notes 11 notes

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119
wbcarts at juno dot com
4 years ago
CONSTANTS and PHP Class Definitions

Using "define('MY_VAR', 'default value')" INSIDE a class definition does not work. You have to use the PHP keyword 'const' and initialize it with a scalar value -- boolean, int, float, or string (no array or other object types) -- 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;         WRONG - Works INSIDE of a class definition.
//const MAX_VALUE = 1.0;         WRONG - Works INSIDE 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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8
ysangkok at gmail dot com
6 months ago
Some answers notes here are outdated, const can be used outside of classes in PHP 5.3+.
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5
gried at NOSPAM dot nsys dot by
7 months 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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29
storm
11 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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1
jacob dot atam at gmail dot com
1 month ago
/*To make a constant case-insensitive add true to the third argument*/

define(name, value, case-insensitive)

eg

Define('FOO','Rasmus ',true);

echo foo;

Rasmus

By default its false.
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7
Raheel Khan
1 year 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
14 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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10
ewspencer at industrex dot com
13 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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3
Andreas R.
9 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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3
hafenator2000 at yahoo dot com
11 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
php at webflips dot net
2 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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