常量

<?php

// 合法的常量名
define("FOO",     "something");
define("FOO2",    "something else");
define("FOO_BAR", "something more");

// 非法的常量名
define("2FOO",    "something");

// 下面的定义是合法的，但应该避免这样做：(自定义常量不要以__开头)
// 也许将来有一天 PHP 会定义一个 __FOO__ 的魔术常量
// 这样就会与你的代码相冲突
define("__FOO__", "something");

?>

注意: 在这里，字母指的是 a-z，A-Z，以及从 128 到 255（0x80-0xff）的 ASCII 字符。

和 superglobals 一样，常量的范围是全局的。常量可以从脚本的任何地方被访问，而不考虑作用域。有关作用域的更多信息请阅读手册中的变量范围。

注意: 从 PHP 7.1.0 开始，类常量可以声明为 protected 或 private 的可见性，使其只在其定义的类的层次范围内可用。

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246

wbcarts at juno dot com ¶

12 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).

down

gried at NOSPAM dot nsys dot by ¶

8 years 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).

down

katana at katana-inc dot com ¶

22 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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warwick dot jm dot barnes at gmail dot com ¶

4 years ago

The documentation says, "You can access constants anywhere in your script without regard to scope", but it's worth keeping in mind that a const declaration must appear in the source file before the place where it's used.

This doesn't work (using PHP 5.4):
<?php
foo();
const X = 1;
function foo() {
    echo "Value of X: " . X;
}
?>
Result: "Value of X: X"

But this works:
<?php
const X = 1;
foo();
function foo() {
    echo "Value of X: " . X;
}
?>
Result: "Value of X: 1"

This is potentially confusing because you can refer to a function that occurs later in your source file, but not a constant. Even though the const declaration is processed at compile time, it behaves a bit like it's being processed at run time.

down

hafenator2000 at yahoo dot com ¶

19 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.

down

ewspencer at industrex dot com ¶

21 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

down

Andreas R. ¶

17 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

down

Raheel Khan ¶

9 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; 
?>

down

storm ¶

19 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.

常量

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