CapÃtulo 13. Constantes

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Constantes predefinidas

Una constante es un identificador para expresar un valor simple. Como el nombre sugiere, este valor no puede variar durante la ejecuciÃ³n del script. (Las constantes especiales __FILE__ y __LINE__ son una excepciÃ³n a esto, ya que actualmente no lo soin). Una constante es sensible a mayÃºsculas por defecto. Por convenciÃ³n, los identificadores de constantes suelen declararse en mayÃºsculas

El nombre de una constante sigue las mismas reglas que cualquier etiqueta en PHP. Un nombre de constante vÃ¡lido empieza con una letra o un caracter de subrayado, seguido por cualquier nÃºmero de letras, nÃºmeros, o subrayados. Se podrÃan expresar mediante la siguiente expresiÃ³n regular: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Nota: Para nuestros propÃ³sitos , entenderemos como letra los carÃ¡cteres a-z, A-Z, y los ASCII del 127 hasta el 255 (0x7f-0xff).

El alcanze de una constante es global, Es decir, es posible acceder a ellas sin preocuparse por el Ã¡mbito de alcance.

Sintaxis

Se puede definir una constante usando la funciÃ³n define(). Una vez definida, no puede ser modificada ni eliminada .

Solo se puede definir como constantes valores escalares (boolean, integer, float y string ).

Para obtener el valor de una constante solo es necesario especificar su nombre. A diferencia de las variables, no se tiene que especificar el prefijo $. Tambien se puede utilizar la funciÃ³n constant(), para obtener el valor de una constante, en el caso de que queramos expresarla de forma dinÃ¡mica Usa la funciÃ³n get_defined_constants() parar obtener una lista de todas las constantes definidas.

Nota: Las contantes y las variables (globales) se encuentran en un espacio de nombres distinto. Esto implica que por ejemplo TRUE y $TRUE son diferentes.

Si usas una constante todavia no definida, PHP asume que estÃ¡s refiriÃ©ndote al nombre de la constante en si. Se lanzarÃ¡ un aviso si esto sucede. Usa la funciÃ³n defined() para comprobar la existencia de dicha constante.

Estas son las diferencias entre constantes y variables:

Las constantes no son precedidas por un sÃmbolo de dolar ($)
Las contantes solo pueden ser definidas usando la funciÃ³n() define , nunca por simple asignaciÃ³n
Las constantes pueden ser definidas y accedidas sin tener en cuenta las reglas de alcanze del Ã¡mbito.
Las constantes no pueden ser redefinidas o eliminadas despues de establecerse; y
Las constantes solo puede albergar valores escalares

Ejemplo 13-1. Definiendo constantes

<?php define("CONSTANT", "Hello world."); echo CONSTANT; // outputs "Hello world." echo Constant; // outputs "Constant" and issues a notice. ?>

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Constantes

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.



http://www.php.net/manual/en/http://indices.com.es/language.types.string.html



--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 (http://www.php.net/manual/en/http://indices.com.es/language.types.string.html) 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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MANUAL DE PHP

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CapÃtulo 13. Constantes

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MANUAL DE PHP

windsurf mercedes camper

CapÃ­tulo 13. Constantes

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CapÃtulo 13. Constantes