Variablen aus externen Quellen
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.
Konstanten
Inhaltsverzeichnis
Eine Konstante ist ein Bezeichner (Name) für einen einfachen Wert. Wie der Name bereits nahelegt, kann der Wert einer Konstanten zur Laufzeit des Skripts nicht verändert werden (ausgenommen die Magischen Konstanten, die aber keine wirklichen Konstanten sind.) Eine Konstante unterscheidet standardmäßig zwischen Groß- und Kleinschreinbung (case-sensitive). Nach gängiger Konvention werden Konstanten immer in Großbuchstaben geschrieben.
Der Name einer Konstanten folgt den gleichen Regeln wie alle anderen Bezeichner in PHP. Ein gültiger Name beginnt mit einem Buchstaben oder einem Unterstrich, gefolgt von beliebig vielen Buchstaben, Ziffern oder Understrichen. Als regulärer Ausdruck könnte das so beschrieben werden: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*
Tipp
Siehe auch Userland Naming Guide.
Beispiel #1 Gültige und ungültige Namen für Konstanten
<?php
// Gueltige Namen fuer Konstanten
define("FOO", "irgendwas");
define("FOO2", "etwas anderes");
define("FOO_BAR", "irgendwas ganz anderes");
// Ungueltige Namen fuer Konstanten
define("2FOO", "irgendwas");
// Folgendes ist korrekt, sollte aber vermieden werden:
// PHP koennte eines Tages eine Magische Konstante unterstuetzen
// die Ihr Skript nicht mehr wie gewuenscht funktionieren laesst
define("__FOO__", "irgendwas");
?>
Hinweis: Für unsere Zwecke ist ein Buchstabe a-z, A-Z und die ASCII-Zeichen von 127 bis 255 (0x7f-0xff).
Wie bei superglobals ist der Gültigkeitsbereich einer Konstanten global. Unabhängig vom Gültigkeitsbereich können Sie in Ihrem Skript überall auf eine Konstante zugreifen. Für mehr Information zum Gültigkeitsbereich lesen Sie bitte den Abschnitt über den Geltungsbereich von Variablen.
add a note
User Contributed Notes
Konstanten - [16 notes]
anj at aps dot anl dot gov ¶
7 years ago
wbcarts at juno dot com ¶
1 year ago
CONSTANTS and PHP Class Definitions
Using "define('MY_VAR', 'default value')" INSIDE a class definition does not work. You have to use the PHP keyword 'const' and initialize it with a scalar value -- boolean, int, float, or string (no array or other object types) -- 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; WRONG - Works INSIDE of a class definition.
//const MAX_VALUE = 1.0; WRONG - Works INSIDE 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).
katana at katana-inc dot com ¶
11 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.
ben at bendodson dot com ¶
5 years ago
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!
hafenator2000 at yahoo dot com ¶
8 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.
Andreas R. ¶
6 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
ewspencer at industrex dot com ¶
9 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
storm ¶
8 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.
martin at larsen dot dk ¶
7 years ago
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] !"
kumar at farmdev ¶
9 years ago
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
$GLOBALS['CP_Lang']['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";
?>
Here are the exact averages of each ab run of 100 requests (averages based on 6 runs):
variable (24.956 secs)
constant (25.426 secs)
array (28.141)
(not huge differences but good to know that using variables won't take a huge performance hit)
tudor at tudorholton dot com ¶
5 years ago
Note that constant name must always be quoted when defined.
e.g.
define('MY_CONST','blah') - correct
define(MY_CONST,'blah') - incorrect
The following error message also indicates this fact:
Notice: Use of undefined constant MY_CONST - assumed 'MY_CONST' in included_script.php on line 5
Note the error message gives you some incorrect information. 'MY_CONST' (with quotes) doesn't actually exist anywhere in your code. The error _is_ that you didn't quote the constant when you defined it in the 'assumed' file.
pdenny at magmic dot com ¶
6 years ago
Note that constants can also be used as default argument values
so the following code:
<?php
define('TEST_CONSTANT','Works!');
function testThis($var=TEST_CONSTANT) {
echo "Passing constants as default values $var";
}
testThis();
?>
will produce :
Passing constants as default values Works!
(I tried this in both PHP 4 and 5)
kencomer at NOSPAM dot kencomer dot com ¶
7 years ago
Being a belt and suspenders person, when I use a constant to do flow control (i.e., using constants to determine which version of a section of the program should be used), I always use something like:
if ( defined('DEBUG') && TRUE===DEBUG )
If you accidentally use DEBUG somewhere before it is defined, PHP will create a new constant called DEBUG with the value 'DEBUG'. Adding the second comparison will prevent the expression from being TRUE when you did not intentionally create the constant. For the constant DEBUG, this would rarely be a problem, but if you had (e.g.) a constant used to determine whether a function was created using case-sensitive comparisons, an accidental creation of the constant IGNORE_CASE having the value 'IGNORE_CASE' could drive you up the wall trying to find out what went wrong, particularly if you had warnings turned off.
In almost all code I write, I put this function definition in my configuration section:
<?php
if (!function_exists("debug_print")) {
if ( defined('DEBUG') && TRUE===DEBUG ) {
function debug_print($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 )
Anonymous ¶
1 year ago
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
$x♂♀◊∆; // using male, female, lozenge, mathematical increment symbol.
?>
You can do this:
<?php
define('≈PI', M_PI); // 180 degrees = π radians ≈ 3.141592654 radians
define('≈180°', M_PI);
define('≈PI÷2', M_PI_2); // 90 degrees ≈ 1.570796327 radians
define('≈90°', M_PI_2);
define('≈PI÷4', M_PI_4); // 45 degrees ≈ 0.785398163 radians
define('≈45°', M_PI_4);
define('≈PI×3÷2', ≈PI+≈PI÷2); // 270 degrees ≈ 4.71238898 radians
define('≈270°', ≈PI×3÷2);
define('≈PI×2', ≈PI*2); // 360 degrees = 2π radians ≈ 6.283185307 radians
define('≈360°', ≈PI×2);
?>
So essentially, you must check each character for acceptance by PHP if you want to use them, but they can really add a semantical value in some cases, thus making your code easier to read and understand..
Angelina Bell ¶
7 years ago
It is so easy to create a constant that the php novice might do so accidently while attempting to call a function with no arguments. For example:
<?php
function LogoutUser(){
// destroy the session, the cookie, and the session ID
blah blah blah;
return true;
}
function SessionCheck(){
blah blah blah;
// check for session timeout
...
if ($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;
?>
meint at meint dot net ¶
2 years ago
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.