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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]*


See also the Userland Naming Guide.

Example #5 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


Note: 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.

User Contributed Notes
ben at bendodson dot com
27-Nov-2007 11:14
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:


('FOO_BAR','It works!');
define('FOO_FOO_BAR','It works again!');

// prints 'It works!'
$changing_variable = 'bar';
constant('FOO_' . strtoupper($changing_variable));

// prints 'It works again!'
$changing_variable = 'foo_bar';
constant('FOO_' . strtoupper($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!
tudor at tudorholton dot com
10-Jul-2007 12:15
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.
Andreas R.
30-Apr-2007 02:19
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'
pdenny at magmic dot com
18-Feb-2007 05:46
Note that constants can also be used as default argument values
so the following code:

testThis($var=TEST_CONSTANT) {
"Passing constants as default values $var";

will produce :

Passing constants as default values Works!

(I tried this in both PHP 4 and 5)
dexen at google dot me dot up
05-Sep-2006 11:02
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', 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 10:24
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 04:42
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

("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 12:38
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 :

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 09:11
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 07:39
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:
function LogoutUser(){
// destroy the session, the cookie, and the session ID
blah blah blah;
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!

"new constant LogoutUser is " . LogoutUser;
27-May-2005 02:23
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')) {
debug_ErrorHandler($errno, $errstr, $errfile, $errline) {
"PHP Error [$errno] [$errstr] at $errline in $errfile.<br>");
hafenator2000 at yahoo dot com
21-Apr-2005 09:09
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.
18-Apr-2005 04:54
An undefined constant evaluates as true when not used correctly. Say for example you had something like this:

// Debug mode


if (
// 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.
12-Jan-2005 09:50
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 08:42
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 12:59
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):
['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";
('Variable1' , "A whole lot of text for this variable as if it were a language string containing a whole lot of text");
['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 01:30
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:


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 02:04
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 04:46
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 04:08
fmmarzoa: In PHP 4.2.2/CLI, I had no problem setting define()'s to the contents of variables:

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

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.


alan at akbkhome dot com
23-Mar-2002 05:08
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
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 07:53
Warning, constants used within the heredoc syntax (https://doc0.ru/phpe/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 05:42
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..

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 01:54
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 12:44
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.

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