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.