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PHP : Language Reference : Constants


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Magic constants

A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for magic constants, which aren't actually constants). A constant is case-sensitive by default. By convention, constant identifiers are always uppercase.

The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thusly: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*


You may also want to take a look at the AppendixT, Userland Naming Guide.

Example4.1.Valid and invalid constant names


// Valid constant names
define("FOO", "something");
define("FOO2", "something else");
define("FOO_BAR", "something more");

// Invalid constant names
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");



For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 127 through 255 (0x7f-0xff).

Like superglobals, the scope of a constant is global. You can access constants anywhere in your script without regard to scope. For more information on scope, read the manual section on variable scope.


You can define a constant by using the define()-function. Once a constant is defined, it can never be changed or undefined.

Only scalar data (boolean, integer, float and string) can be contained in constants. Do not define resource constants.

You can get the value of a constant by simply specifying its name. Unlike with variables, you should not prepend a constant with a $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name dynamically. Use get_defined_constants() to get a list of all defined constants.


Constants and (global) variables are in a different namespace. This implies that for example TRUE and $TRUE are generally different.

If you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a string (CONSTANT vs "CONSTANT"). An error of level E_NOTICE will be issued when this happens. See also the manual entry on why $foo[bar] is wrong (unless you first define() bar as a constant). If you simply want to check if a constant is set, use the defined() function.

These are the differences between constants and variables:

  • Constants do not have a dollar sign ($) before them;
  • Constants may only be defined using the define() function, not by simple assignment;
  • Constants may be defined and accessed anywhere without regard to variable scoping rules;
  • Constants may not be redefined or undefined once they have been set; and
  • Constants may only evaluate to scalar values.

Example4.2.Defining Constants

("CONSTANT", "Hello world.");
CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.

See also Class Constants.

Related Examples ( Source code ) » language.constants

Code Examples / Notes » language.constants


Warning, constants used within the heredoc syntax ( 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

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


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 __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

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);


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..
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.


Re: Storm.
I ran that code (in PHP4)
if (DEBUG) {
  // echo some sensitive data.
and saw this warning:
"Use of undefined constant DEBUG - assumed 'DEBUG'"
A clearer workaround is to use
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:
if (defined('DEBUG')) {
function debug_ErrorHandler($errno, $errstr, $errfile, $errline) {
 print("PHP Error [$errno] [$errstr] at $errline in $errfile.


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.


Note that constants can also be used as default argument values
so the following code:
 function testThis($var=TEST_CONSTANT) {
  echo "Passing constants as default values $var";
will produce :
Passing constants as default values Works!
(I tried this in both PHP 4 and 5)


Note that constant name must always be quoted when defined.
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.


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.

a dot eibach

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.


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.

angelina bell

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:
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!
print "new constant LogoutUser is " . LogoutUser;


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
   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.

mike powell

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.

andreas r.

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:
while the latter two are also mentioned in 'Directory Functions'


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]);


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] !"


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:
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


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.


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 :
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 )


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):
$GLOBALS['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";
define('Variable1' , "A whole lot of text for this variable as if it were a language string containing a whole lot of text");
$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)


An undefined constant evaluates as true when not used correctly. Say for example you had something like this:
// Debug mode
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:
// Debug mode
if (DEBUG == 1) {
  // echo some sensitive data.
Now it works correctly.


Ah, I forgot to point that out in my previous note:
> "Constants may only evaluate to scalar values."
Currently, resources are "abstract datatypes based on integers".
Though this may change in the future, and is_scalar() rejects resources as non-scalar, you can define resource constants, like
<?php define('DB', mysql_connect());?>
(think of STDIN, STDOUT and STDERR).
Resource constants will still remain resources; try var_dump().


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!)


> "A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores."
This rules only applies when using constants like <?php echo CONSTANT;?> to not confuse the parser - PHP will be happy with any string for a constant name (define).
define('Red herring!', '<°)))><');
#echo Red herring!;
// The above example will not work, the parser will get
// angry seeing a whitespace where it shouldn’t be.
// Let’s have that with constant():
echo constant('Red herring!'); // <°)))><
You could go on with newlines, tabs and stuff... that is, however, crude coding style.

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