define

Be aware that if "Notice"-level error reporting is turned off, then trying to use a constant as a variable will result in it being interpreted as a string, if it has not been defined.I was working on a program which included a config file which contained:<?phpdefine('ENABLE_UPLOADS', true);?>Since I wanted to remove the ability for uploads, I changed the file to read:<?php//define('ENABLE_UPLOADS', true);?>However, to my surprise, the program was still allowing uploads. Digging deeper into the code, I discovered this:<?phpif ( ENABLE_UPLOADS ):?>Since 'ENABLE_UPLOADS' was not defined as a constant, PHP was interpreting its use as a string constant, which of course evaluates as True.

Not sure why the docs omit this, but when attempting to define() a constant that has already been defined, it will fail, trigger an E_NOTICE and the constant's value will remain as it was originally defined (with the new value ignored).(Guess that's why they're called "constants".)

define() will define constants exactly as specified.  So, if you want to define a constant in a namespace, you will need to specify the namespace in your call to define(), even if you're calling define() from within a namespace.  The following examples will make it clear.The following code will define the constant "MESSAGE" in the global namespace (i.e. "\MESSAGE").<?phpnamespace test;define('MESSAGE', 'Hello world!');?>The following code will define two constants in the "test" namespace.<?phpnamespace test;define('test\HELLO', 'Hello world!');define(__NAMESPACE__ . '\GOODBYE', 'Goodbye cruel world!');?>

With php 7 you can now define arrays. consider the following code:<?phpdefine( "EXPLENATIVES", [1 => "Foo Bar", 2 => "Fehw Bahr", 3 => "Foo Bahr", 4 => "Fooh Bar", 5 => "Fooh Bhar", 6 => "Foo Barr", 7 => "Foogh Bar", 8 => "Fehw Barr", 9 => "Fu bar", 10 => "Foo Bahr", 11 => "Phoo Bar", 12 => "Foo Bawr", 13 => "Phooh Baughr", 14 => "Foogan Bargan", 15 => "Foo Bahre", 16 => "Fu Bahar", 17 => "Fugh Bar", 18 => "Phou Baughr"]);//set up define methods using mixed values; both array and non-array valuesdefine("NAVBTNS", [EXPLENATIVES, "Nouns", "Verbs", "Adjectives"]);//function to create a dropdown menu using the EXPLENATIVES array $btn=EXPLENATIVES=assoc_arrayfunction construct_navbar_buttons(){        $btns = '<ul class="nav navbar-nav">';                            foreach(NAVBTNS as $button => $btn){        if(is_array($btn)){            $btns .= '<li class="dropdown">                        <a class="dropdown-toggle" data-toggle="dropdown" href="?id='.$btn.'">                            <i class="glyphicon glyphicon-user"></i> You Dare Say? <i class="glyphicon glyphicon-collapse-down"></i>                        </a>                        <ul class="dropdown-menu dropdown-user">';            foreach(EXPLENATIVES as $key => $button){                $btns .= '<li><a href="#">'.$button.'</a></li>';            }            $btns .= '</ul>';        }else{                $btns .= '<li><a href="#">'.$btn.'</a></li>';        }    }        $btns .= '</ul>';    return $btns;}            Love this new implementation!?>

Found something interesting.  The following define:

<?php
define("THIS-IS-A-TEST","This is a test");
echo THIS-IS-A-TEST;
?>

Will return a '0'.

Whereas this:

<?php
define("THIS_IS_A_TEST","This is a test");
echo THIS_IS_A_TEST;
?>

Will return 'This is a test'.

This may be common knowledge but I only found out a few minutes ago.

[EDIT BY danbrown AT php DOT net: The original poster is referring to the hyphens versus underscores.  Hyphens do not work in defines or variables, which is expected behavior.]

The value of a constant can be the value of another constant.<?phpdefine("NEW_GOOD_NAME_CONSTANT", "I have a value");define("OLD_BAD_NAME_CONSTANT", NEW_GOOD_NAME_CONSTANT);echo NEW_GOOD_NAME_CONSTANT; // currentecho OLD_BAD_NAME_CONSTANT; // legacy?>

This is obvious, but easy to forget: if you include a file, the include file can only make use of constants already defined. For example:

<?php
define("VEG","cabbage");
require("another file");
define("FRUIT","apple");

// "another file":
echo VEG; // cabbage
echo FRUIT; // FRUIT
?>

I think worth mentioning is that define() appears to ignore invalid constant names.One immediate implication of this seem to be that if you use an invalid constant name you have to use constant() to access it and obviously that you can't use the return value from define() to tell you whether the constant name used is invalid or not.For example:$name = '7(/!§%';var_dump(define($name, "hello")); // outputs bool(true)var_dump(constant($name)); // outputs string(5) "hello"

Php 7 - Define: "Defines a named constant at runtime. In PHP 7, array values are also accepted."But prior PHP 7, you can maybe do this, to pass an array elsewhere using define:$to_define_array = serialize($array);define( "DEFINEANARRAY", $to_define_array );... and so ...$serialized = DEFINEANARRAY; // passing directly the defined will not work      $our_array = unserialize($serialized); print_r($our_array);

There's an undocumented side-effect of setting the third parameter to true (case-insensitive constants): these constants can actually be "redefined" as case-sensitive, unless it's all lowercase (which you shouldn't define anyway).The fact is that case-sensitive constants are stored as is, while case-insensitive constants are stored in lowercase, internally. You're still allowed to define other constants with the same name but capitalized differently (except for all lowercase).<?php  // "echo CONST" prints 1, same as "echo const", "echo CoNst", etc.  define('CONST', 1, true);  echo CONST; // Prints 1  define('CONST', 2);  echo CONST; // Prints 2  echo CoNsT; // Prints 1  echo const; // Prints 1  // ** PHP NOTICE: Constant const already defined **   define('const', 3);  echo const; // Prints 1  echo CONST; // Prints 2?>Why would you use this?A third party plugin might attempt to define a constant for which you already set a value. If it's fine for them to set the new value, assuming you cannot edit the plugin, you could define your constant case-insensitive. You can still access the original value, if needed, by using any capitalization other than the one the plugin uses. As a matter of fact, I can't think of another case where you would want a case-insensitive constant...

You can define constants with variable names (works also with constant values or variables or array values or class properties and so on - as long it's a valid constant name).<?php        # Define a constant and set a valid constant name as string value    define("SOME_CONSTANT", "NEW_CONSTANT");        # Define a second constant with dynamic name (the value from SOME_CONSTANT)    define(SOME_CONSTANT, "Some value");        # Output    echo SOME_CONSTANT; // prints "NEW_CONSTANT"    echo "<br>";    echo NEW_CONSTANT; // prints "Some value"        ?>Needless to say that you'll lose your IDE support for refactoring and highlighting completely for such cases.No clue why someone would / could actually use this but i thought it's worth mentioning.