章 13. 常量

目录
语法
魔术常量

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。按照惯例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]*

例子 13-1. 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"); // This is valid, but should be avoided: // PHP may one day provide a magical constant // that will break your script define("__FOO__", "something"); ?>

注: 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。

语法

可以用 define() 函数来定义常量。一个常量一旦被定义，就不能再改变或者取消定义。

常量只能包含标量数据（boolean，integer，float 和 string）。

可以简单的通过指定其名字来取得常量的值，不要在常量前面加上 $ 符号。如果常量名是动态的，也可以用函数 constant() 来读取常量的值。用 get_defined_constants() 可以获得所有已定义的常量列表。

注: 常量和（全局）变量在不同的名字空间中。这意味着例如 TRUE 和 $TRUE 是不同的。

如果使用了一个未定义的常量，PHP 假定想要的是该常量本身的名字，如同用字符串调用它一样（CONSTANT 对应 "CONSTANT"）。此时将发出一个 E_NOTICE 级的错误。参见手册中为什么 $foo[bar] 是错误的（除非事先用 define() 将 bar 定义为一个常量）。如果只想检查是否定义了某常量，用 defined() 函数。

常量和变量不同：

常量前面没有美元符号（$）；
常量只能用 define() 函数定义，而不能通过赋值语句；
常量可以不用理会变量范围的规则而在任何地方定义和访问；
常量一旦定义就不能被重新定义或者取消定义；
常量的值只能是标量。

例子 13-2. 定义常量
<?php define("CONSTANT", "Hello world."); echo CONSTANT; // outputs "Hello world." echo Constant; // outputs "Constant" and issues a notice. ?>

参见类常量。

add a note User Contributed Notes

dexen at google dot me dot up
05-Sep-2006 07:02


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 06:24


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
21-Dec-2005 12:42


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 08:38


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:



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 :



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 :



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 )

a dot eibach at gmx dot net
01-Sep-2005 05:11


It took me almost 30 minutes to find out what was wrong in my code. I thought I had defined all constants correctly: correct quotes, and whatnot.

The problem: I am a C programmer and I used #define with the preprocessor hash sign! No effect, naturally.

So if you happen to come from C world and you program PHP, *DO NOT* use the preprocessor hash as you're used to in C.

Angelina Bell
26-Jul-2005 03:39


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;

?>

ck
27-May-2005 10:23


Re: Storm.

I ran that code (in PHP4)

<?php

if (DEBUG) {

   // echo some sensitive data.

}

?>

 and saw this warning:

"Use of undefined constant DEBUG - assumed 'DEBUG'"



A clearer workaround is to use

<?php

if (defined('DEBUG')) {

   // echo some sensitive data.

}

?>

Thanks for pointing out this big gotcha.



Another reason to turn on warnings during testing.  Good web servers are set up to suppress warning and error output to the browser, so this is handy:

<?php

if (defined('DEBUG')) {

  error_reporting(E_ALL);

  set_error_handler('debug_ErrorHandler');

}

function debug_ErrorHandler($errno, $errstr, $errfile, $errline) {

  print("PHP Error [$errno] [$errstr] at $errline in $errfile.<br>");

}

?>

hafenator2000 at yahoo dot com
22-Apr-2005 05:09


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
19-Apr-2005 12:54


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.

Charles
12-Jan-2005 05:50


To clarify from the previous post:



When you define a constant, it becomes fixed at that point and is immutable. You can add variables - but the constant becomes the contents of that variable when the define is evaluated. If you try:



define( "_A_TEXT" , "The value is " . $arr[$i] );



It would be evaluated ONCE with the current value of the $i index of array $arr. As the post pointed out, this is probably not what you want. You can easily create:



define( "_A_TEXT" , "The value is ");

....

echo _A_TEXT . $arr[$i];



Which would give you what you wanted: the constant string with the contents of the array appended.

the_coder at colina2004 dot com
25-Jun-2004 04:42


I'm currently working on a site that has got to have two languages, and I wanted to use define's in functions to make everything simpler.



However, I ran into a problem. PHP doesn't recognize the variable in:

define("constantName", "This is an array variable - {$array[$i][2]}");



I can't use that in a for cycle, like I wanted to:



for ($i = 0; $i < count($array); $i++) {

echo constantName . "<br />"

}



The method I found (I think it's been mentioned before) is to:



define("constantName", "This is an array variable - %s");



And then:



for ($i = 0; $i < count($array); $i++) {

printf(constantName, $array[$i][2]);

}

kumar at farmdev
26-Oct-2003 08:59


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 09:30


I find using the concatenation operator helps disambiguate value assignments with constants. For example, setting constants in a global configuration file:



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:



require_once(FUNCTIONS."/database.fnc");

require_once(FUNCTIONS."/randchar.fnc");



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



$userid  = randchar(8,'anc','u');

$usermap = USERDIR."/".$userid.".png";



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



-- Erich

php-comment-2003-july-24 at ryandesign dot de
24-Jul-2003 10:04


Late reply to fmmarzoa at gmx dot net: You're better off using sprintf format and defining your strings like this:



define('strArticleDescr', 'Published by %1$s on %2$s in %2$s');



It's more standard than what you're doing. Then instead of outputting it using an eval, do this:



echo sprintf(strArticleDescr, $article_author, $article_date, $article_lang_name');



And even better for i18n and l10n, don't use defines; use gettext. See the PHP manual section on gettext and the GNU gettext website. Gettext requires some modification of the way you think about strings but I find it worthwhile to make that adjustment.

Mike Powell
25-Mar-2003 12:46


In response to the notes above about variable references in constants, double quotes isn't a proper solution because it parses the variable at the time the constant is defined. The desired behavior is to have the variables parsed at the time the constant is referenced, and this behavior can really only be achieved by using eval(), as described above.

gv (at) damnsw (dot) net
06-Nov-2002 12:08


fmmarzoa: In PHP 4.2.2/CLI, I had no problem setting define()'s to the contents of variables:



<?

    $foo = "PHP";

    define( "bar", "$foo is a good thing." );

    print bar;

?>



Will print "PHP is a good thing.".



A notable difference, however, between my example and yours is your use of single-quotes.  Strings in single quotes (') will not be expanded: 



print '$foo'; 



Will print '$foo', not the contents of $foo.



http://www.php.net/manual/en/language.types.string.php



--gv

alan at akbkhome dot com
24-Mar-2002 01:08


The __FILE__ constant in 4.2rc1 (CLI) will return the location of script specified to be run, rather than the absolute file.



eg. /usr/bin/phpmole (a softlink to /usr/lib/php/phpmole/phpmole.php)



started like this

bash#/usr/bin/phpmole 

 

the line echo __FILE__ in phpmole.php will output /usr/bin/phpmole - in the CGI it would have returned /usr/lib/php/phpmole/phpmole.php



the workaround is to check for links!!

$f = __FILE__;

if (is_link($f)) $f = readlink($f);

katana at katana-inc dot com
26-Feb-2002 03:53


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.

afuse at yahoo dot com
11-Jun-2001 01:42


The pre-defined constant '__FILE__' does not work in same way at every version of PHP.





Some version of PHP has the relative path, and some other has the absolute path on __FILE__ constant..





Please be carefull in use..





[PS]


I have not tested at all versions of PHP but the version of 4.04pl.. and 4.05 are certainly not working in same way..  If you want to see that bug(?), I can show you an example.

silvein at sonique dot com
24-Jan-2001 09:54


It may be useful to note that, in php4 (what version this started I don't know, but it didn't do it before we upgraded to php4) __FILE__ will follow symlinks to the origional file.

tom dot harris at home dot com
05-Aug-2000 08:44


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.

add a note