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常量

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常量是一个简单值的标识符（名字）。如同其名称所暗示的，在脚本执行期间该值不能改变（除了所谓的魔术常量，它们其实不是常量）。常量默认为大小写敏感。通常常量标识符总是大写的。

常量名和其它任何 PHP 标签遵循同样的命名规则。合法的常量名以字母或下划线开始，后面跟着任何字母，数字或下划线。用正则表达式是这样表达的：[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Tip

请参见Userland Naming Guide。

Example #1 合法与非法的常量名

<?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");

?>

Note: 在这里，字母是 a-z，A-Z，以及从 127 到 255（0x7f-0xff）的 ASCII 字符。

和 superglobals 一样，常量的范围是全局的。不用管作用域就可以在脚本的任何地方访问常量。有关作用得更多信息请阅读手册中的变量范围。

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PHP手册 - N: 常量

用户评论:

Anonymous (27-Aug-2011 03:35)

PHP will allow characters other than those shown for both variable names and constants, and therefore probably functin names, too.  And I'm pretty sure array indexes also.

Many others are allowed, while some are not.  It seems like a craps-shoot at first, but there is a 'bit' of reason...

you can do this:
<?php
$x?; // using the "acute (reverse) accent mark"  (#182)
?>
but not:
<?php
$x′;  // using the "prime mark"  (#8242)
?>

but you can do:
<?php
$x??±√2×π  // using double dagger, dagger, plus-minus, square-root, the number 2, the "times symbol" and the greek letter pi (lowercase).
?>
but not:
<?php
$x♂♀??;  // using male, female, lozenge, mathematical increment symbol.
?>

You can do this:
<?php
define('≈PI',   M_PI);        // 180 degrees = π radians ≈ 3.141592654 radians
define('≈180°', M_PI);
define('≈PI÷2', M_PI_2);      // 90 degrees ≈ 1.570796327 radians
define('≈90°',  M_PI_2);
define('≈PI÷4', M_PI_4);      // 45 degrees ≈ 0.785398163 radians
define('≈45°',  M_PI_4);
define('≈PI×3÷2', ≈PI+≈PI÷2); // 270 degrees ≈ 4.71238898 radians
define('≈270°', ≈PI×3÷2);
define('≈PI×2', ≈PI*2);       // 360 degrees = 2π radians ≈ 6.283185307 radians
define('≈360°', ≈PI×2);
?>

So essentially, you must check each character for acceptance by PHP if you want to use them, but they can really add a semantical value in some cases, thus making your code easier to read and understand..

meint at meint dot net (11-Jan-2011 08:08)

A nice way to set and verify a constant is not already set:

defined('CONSTANT') or define('CONSTANT', 'value');

If the constant is defined the expression resolves to false, if the constant isn't set it will be defined.

ben at bendodson dot com (27-Nov-2007 11:14)

I recently found I needed a way of retrieving the value of a constant dynamically - e.g. trying to find the value of FOO_BAR by passing 'FOO_' . $someVariableWithValueBAR.  I came up with the following solution:

<?php

define('FOO_BAR','It works!');
define('FOO_FOO_BAR','It works again!');

// prints 'It works!'
$changing_variable = 'bar';
echo constant('FOO_' . strtoupper($changing_variable));

// prints 'It works again!'
$changing_variable = 'foo_bar';
echo constant('FOO_' . strtoupper($changing_variable));

?>

Note the use of strtoupper() as constants should be defined in uppercase for good practice - feel free to remove if you have constants defined in lowercase or you can set $changing_variable as uppercase.

Might be of some use to someone!

tudor at tudorholton dot com (10-Jul-2007 01:15)

Note that constant name must always be quoted when defined.

e.g.
define('MY_CONST','blah') - correct
define(MY_CONST,'blah') - incorrect

The following error message also indicates this fact:
Notice:  Use of undefined constant MY_CONST - assumed 'MY_CONST' in included_script.php on line 5

Note the error message gives you some incorrect information.   'MY_CONST' (with quotes) doesn't actually exist anywhere in your code.  The error _is_ that you didn't quote the constant when you defined it in the 'assumed' file.

Andreas R. (30-Apr-2007 03:19)

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

pdenny at magmic dot com (18-Feb-2007 05:46)

Note that constants can also be used as default argument values
so the following code:

<?php
  define('TEST_CONSTANT','Works!');
  function testThis($var=TEST_CONSTANT) {
      echo "Passing constants as default values $var";
  }
  testThis();
?>

will produce :

Passing constants as default values Works!

(I tried this in both PHP 4 and 5)

dexen at google dot me dot up (05-Sep-2006 12:02)

1) Constants are invaluable when you want to be sure that *nobody*  changes your important piece of data through lifetime of script -- especially when you're developing in team -- as this can cause strange, hard to track bugs.

2) Using constants is prefered over ``magic values'', as it leads to self-documenting code. Also saves you from scanning and tweaking tens of files should the value ever change.
Consider example: <?php
if ( $headers['code'] = 505 ) { //wth is 505? What do following code do? ?>
versus: <?php
if ( $headers['code'] = HTTP_VERSION_NOT_SUPPORTED ) {
   $this->useHttp = '1.0'; ?>

In response to ``kencomer'':
3) Why not to use <?php
define( 'DEBUG', FALSE );
define( 'DEBUG', TRUE ); ?>
and comment one of them out as needed when developing/deploying?
That'd save a lot of ugly ``if ( defined( 'DEBUG' ) && DEBUG ) {}''.

4) For debugging toggled on/off you pretty often want to use assert() anyway. You're free to turn it on/off at any moment (thou you better do it only once ;) ). assert() gives some nice details upon failed assertion, like file/line/function and context (that's invaluable!)

martin at larsen dot dk (23-Feb-2006 10:24)

I find variables much more flexible than constants because variables can be used inside quotes and heredocs etc. Especially for language systems, this is nice.

As stated in one of the previous notes, there is no speed penalty by using variables. However, one issue is that you risc name collision with existing variables. When implementing a language system I simply found that adding a prefix to all the variables was the way to go, for example:

$LNG_myvar1 = "my value";

That is easier and performs faster than using arrays like

$LNG['myvar'] = "my value";

As a final note, implementing a new superglobal in PHP would make using constants much more beneficial. Then it could be used in qoutes like this:

"The constant myconst has the value $CONSTANTS[myconst] !"

anj at aps dot anl dot gov (20-Dec-2005 04:42)

It is possible to define constants that have the same name as a built-in PHP keyword, although subsequent attempts to actually use these constants will cause a parse error. For example in PHP 5.1.1, this code

    <?php
    define("PUBLIC", "Hello, world!");
    echo PUBLIC;
    ?>

gives the error

    Parse error: syntax error, unexpected T_PUBLIC in test.php on line 3

This is a problem to be aware of when converting PHP4 applications to PHP5, since that release introduced several new keywords that used to be legal names for constants.

kencomer at NOSPAM dot kencomer dot com (14-Sep-2005 01:38)

Being a belt and suspenders person, when I use a constant to do flow control (i.e., using constants to determine which version of a section of the program should be used), I always use something like:

if ( defined('DEBUG') && TRUE===DEBUG )

If you accidentally use DEBUG somewhere before it is defined, PHP will create a new constant called DEBUG with the value 'DEBUG'. Adding the second comparison will prevent the expression from being TRUE when you did not intentionally create the constant. For the constant DEBUG, this would rarely be a problem, but if you had (e.g.) a constant used to determine whether a function was created using case-sensitive comparisons, an accidental creation of the constant IGNORE_CASE having the value 'IGNORE_CASE' could drive you up the wall trying to find out what went wrong, particularly if you had warnings turned off.

In almost all code I write, I put this function definition in my configuration section:

<?php
if (!function_exists("debug_print")) {
  if ( defined('DEBUG') && TRUE===DEBUG ) {
    function debug_print($string,$flag=NULL) {
      /* if second argument is absent or TRUE, print */
      if ( !(FALSE===$flag) )
        print 'DEBUG: '.$string . "\n";
    }
  } else {
    function debug_print($string,$flag=NULL) {
    }
  }
}
?>

Then, in my code, I'll sprinkle liberal doses of debug code like :

<?php
define("DEBUG_TRACK_EXAMPLE_CREATION",FALSE);
class Example extends Something {
  __construct($whatever) {
    debug_print( "new instance of Example created with '$whatever'\n",DEBUG_TRACK_EXAMPLE_CREATION);
  }
}
?>

and :

<?php
debug_print("finished init.\n");
?>

In the first case, I would not want to see that message every time I went into DEBUG mode, so I made it a special case. The second case is always printed in DEBUG mode. If I decide to turn everything on, special cases and all, all I have to do is comment out the "if" line in debug_print() and presto magicko! It costs a little and gains a lot.

As another belt-and-suspenders aside, notice that, unlike most people, I put the language constant (e.g.,TRUE, "string", etc.) on the left side of the comparison. By doing that, you can never accidentally do something like
  if ( $hard_to_find_error="here" )

because you always write it as
  if ( "here"==$no_error )

or, if you got it wrong,
  if ( "here"=$easy_to_find_parse_error )

Angelina Bell (25-Jul-2005 08:39)

It is so easy to create a constant that the php novice might do so accidently while attempting to call a function with no arguments.  For example:
<?php
function LogoutUser(){
// destroy the session, the cookie, and the session ID
  blah blah blah;
  return true;
}
function SessionCheck(){
  blah blah blah;
// check for session timeout
...
    if ($timeout) LogoutUser;  // should be LogoutUser();
}
?>

OOPS!  I don't notice my typo, the SessionCheck function
doesn't work, and it takes me all afternoon to figure out why not!

<?php
LogoutUser;
print "new constant LogoutUser is " . LogoutUser;
?>

hafenator2000 at yahoo dot com (21-Apr-2005 10:09)

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.

storm (18-Apr-2005 05:54)

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.

kumar at farmdev (26-Oct-2003 01:59)

before embarking on creating a language system I wanted to see if there was any speed advantage to defining language strings as constants vs. variables or array items.  It is more logical to define language strings as constants but you have more flexibility using variables or arrays in your code (i.e. they can be accessed directly, concatenated, used in quotes, used in heredocs whereas constants can only be accessed directly or concatenated).

Results of the test:
declaring as $Variable is fastest
declaring with define() is second fastest
declaring as $Array['Item'] is slowest

=======================================
the test was done using PHP 4.3.2, Apache 1.3.27, and the ab (apache bench) tool.
100 requests (1 concurrent) were sent to one php file that includes 15 php files each containing 100 unique declarations of a language string.

Example of each declaration ("Variable" numbered 1 - 1500):
<?php
$GLOBALS['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";
?>
<?php
define('Variable1' , "A whole lot of text for this variable as if it were a language string containing a whole lot of text");
?>
<?php
$GLOBALS['CP_Lang']['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";
?>

Here are the exact averages of each ab run of 100 requests (averages based on 6 runs):
variable (24.956 secs)
constant (25.426 secs)
array (28.141)

(not huge differences but good to know that using variables won't take a huge performance hit)

ewspencer at industrex dot com (18-Aug-2003 02:30)

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

katana at katana-inc dot com (25-Feb-2002 07:53)

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.

tom dot harris at home dot com (05-Aug-2000 01:44)

To get a full path (the equivalent of something like "__PATH__") use
dirname($SCRIPT_FILENAME)
to get the directory name of the called script and
dirname(__FILE__)
to get the directory name of the include file.