Глава 13. Константы

Содержание
Синтаксис
Предопределенные константы

Константы - это идентификаторы простых значений. Исходя из их названия, нетрудно понять, что их значение не может изменяться в ходе выполнения скрипта (исключения представляют "волшебные" константы, которые на самом деле не являются константами в полном смысле этого слова). Имена констант чувствительны к регистру. По принятому соглашению, имена констант всегда пишутся в верхнем регистре.

Имя константы должно соответствовать тем же правилам, которыми руководствуются и другие имена в PHP. Правильное имя начинается с буквы или символа подчеркивания и состоит из букв, цифр и подчеркиваний. Регулярное выражение для проверки правильности имени константы выглядит так: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Замечание: Понятие "буквы" здесь - это символы a-z, A-Z, и другие символы с ASCII-кодами от 127 до 255 (0x7f-0xff).

Как и суперглобальные переменные, константы доступны из любой области видимости. Вы можете использовать константы в любом месте вашего скрипта, не обращая внимания на текущую область видимости. Подробную информацию об областях видимости можно найти здесь.

Синтаксис

Вы можете определить константу с помощью функции define(). После того, как константа определена, ее значение не может быть изменено или аннулировано.

Константы могут содержать только скалярные данные (логического, целого, плавающего и строкового типов).

Получить значение константы можно, указав ее имя. В отличие от переменных, вам не потребуется предварять имя константы символом $. Также вы можете использовать функцию constant() для получения значения константы, если вы формируете имя константы динамически. Используйте функцию get_defined_constants() для получения списка всех объявленных констант.

Замечание: Константы и (глобальные) переменные находятся в разном пространстве имен. Это означает, что, например, TRUE и $TRUE являются совершенно разными вещами.

Если вы используете неопределенную константу, PHP предполагает, что вы имеете ввиду само имя константы, как если бы вы указали переменную типа строка (CONSTANT и "CONSTANT"). При этом будет сгенерирована ошибка типа E_NOTICE. Смотрите также главу руководства, которая разъясняет, почему $foo[bar] - это неправильно (конечно, если вы перед этим не объявили bar как константу с помощью define()). Если вы просто хотите проверить, определена ли константа, используйте функцию defined().

Различия между константами и переменными:

  • У констант нет приставки в виде знака доллара ($);

  • Константы можно определить только с помощью функции define(), а не присваиванием значения;

  • Константы могут быть определены и доступны в любом месте без учета области видимости;

  • Константы не могут быть определены или аннулированы после первоначального объявления; и

  • Константы могут иметь только скалярные значения.

Пример 13-1. Определение констант

<?php
define("CONSTANT", "Здравствуй, мир.");
echo CONSTANT; // выводит "Здравствуй, мир."
echo Constant; // выводит "Constant" и выводит предупреждениее.
?>



Константы
martin at larsen dot dk
23-Feb-2006 02:24
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 08:42
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 05:38
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"=$easy_to_find_parse_error )
a dot eibach at gmx dot net
01-Sep-2005 02:11
It took me almost 30 minutes to find out what was wrong in my code. I thought I had defined all constants correctly: correct quotes, and whatnot.
The problem: I am a C programmer and I used #define with the preprocessor hash sign! No effect, naturally.
So if you happen to come from C world and you program PHP, *DO NOT* use the preprocessor hash as you're used to in C.
Angelina Bell
25-Jul-2005 12:39
It is so easy to create a constant that the php novice might do so accidently while attempting to call a function with no arguments.  For example:
<?php
function LogoutUser(){
// destroy the session, the cookie, and the session ID
  blah blah blah;
  return true;
}
function SessionCheck(){
  blah blah blah;
// check for session timeout
...
   if ($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 07:23
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
21-Apr-2005 02:09
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 09:54
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 01:50
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
24-Jun-2004 01:42
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
25-Oct-2003 05:59
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 06:30
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 07:04
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
24-Mar-2003 08:46
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
05-Nov-2002 08:08
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.

manual/en/language.types.string.php

--gv
alan at akbkhome dot com
23-Mar-2002 09:08
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
25-Feb-2002 11:53
Warning, constants used within the heredoc syntax (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
10-Jun-2001 10:42
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
23-Jan-2001 05:54
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
04-Aug-2000 05:44
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.

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 Last updated: Tue, 15 Nov 2005