PHP | Constants

Constants are either identifiers or simple names that can be assigned any fixed values. They are similar to a variable except that they can never be changed. They remain constant throughout the program and cannot be altered during execution. Once a constant is defined, it cannot be undefined or redefined. Constant identifiers should be written in upper case following the convention. By default, a constant is always case-sensitive, unless mentioned. A constant name must never start with a number. It always starts with a letter or underscores, followed by letter, numbers or underscore. It should not contain any special characters except underscore, as mentioned.

Creating a PHP Constant

The define() function in PHP is used to create a constant as shown below:
Syntax:

define(name, value, case_insensitive)

The parameters are as follows:

• name: The name of the constant.
• value: The value to be stored in the constant.
• case_insensitive: Defines whether a constant is case insensitive. By default this value is False, i.e., case sensitive.

Example:

<?php

// This creates a case-sensitive constant
define("WELCOME", "GeeksforGeeks");
echo WELCOME, "\n";

// This creates a case-insensitive constant
define("HELLO", "GeeksforGeeks", true);
echo hello;

?>

Output:

GeeksforGeeks
GeeksforGeeks

constant() function

Instead of using the echo statement ,there is an another way to print constants using the constant() function.

Syntax

constant(name)

Example:

<?php

   define("WELCOME", "GeeksforGeeks!!!");

   echo WELCOME, "\n";

   echo constant("WELCOME");
   // same as previous

?>

Output:

GeeksforGeeks!!!
GeeksforGeeks!!!

Constants are Global: By default, constants are automatically global, and can be used throughout the script, accessible inside and outside of any function.
Example:

<?php

define("WELCOME", "GeeksforGeeks");

function testGlobal() {
    echo WELCOME;
}

testGlobal();

?>

GeeksforGeeks

Constants vs Variables

• A constant, once defined can never be undefined but a variable can be easily undefined.
• There is no need to use dollar sign($) before constants during assignment but while declaring variables we use a dollar sign.
• A constant can only be defined using a define() function and not by any simple assignment.
• Constants dont need to follow any variable scoping rules and can be defined anywhere.

This article is contributed by Chinmoy Lenka. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.