Submit a Pull Request Report a Bug

    Syntax

    You can define a constant by using the define()-function or by using the const keyword outside a class definition as of PHP 5.3.0. 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. It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results.

    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.

    Note: 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.

    Eksempel #1 Defining Constants

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

    Eksempel #2 Defining Constants using the const keyword

    <?php
    // Works as of PHP 5.3.0
    const CONSTANT = 'Hello World';

    echo     CONSTANT;
    ?>

    Note:

    As opposed to defining constants using define(), constants defined using the const keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops or if statements.

    See also Class Constants.

    add a note add a note

    User Contributed Notes 4 notes

    up
    10
    login at (two)view dot de
    6 years ago
    Just a quick note:
    From PHP7 on you can even define a multidimensional Array as Constant:

    define('QUARTLIST',array('1. Quarter'=>array('jan','feb','mar'),'2.Quarter'=>array('may','jun','jul'));

    does work as expected.
    up
    -5
    halfhearted13 at gmail dot com
    3 years ago
    Constants of constant :

    <?php

    const foo = hello;
    const     bar = foo;
    const     bd = bar;

    echo     "bd = " .bd;   // Prints out: Hello

    ?>
    up
    -8
    privat at ulrich-kapp dot de
    4 years ago
    const ArrayFromTextfile = file("mytextfile.txt", FILE_IGNORE_NEW_LINES);

    does not work, it throws an error:
    Fatal error: Constant expression contains invalid operations in php shell code on line …

    Instead use:

    define ("ArrayFromTextfile", file("mytextfile.txt", FILE_IGNORE_NEW_LINES));

    print_r(ArrayFromTextfile);

    Result:
    Array
    (
        [0] ⇨ Line 1
        [1] ⇨ Line 2
        [2] ⇨ Line 3
        [3] => …
    )
    up
    -12
    mh at hanft dot de
    4 years ago
    With PHP 5.6, multi-dimensional arrays are also possible if you use "const" instead of "define". So,

    define('QUARTLIST',array('1. Quarter'=>array('jan','feb','mar'),'2.Quarter'=>array('may','jun','jul')));

    won't work with PHP 5.6, but

    const QUARTLIST=array('1. Quarter'=>array('jan','feb','mar'),'2.Quarter'=>array('may','jun','jul'));

    will.
    add a note add a note
    To Top