PHP Constants

In this lesson, you’ll learn about PHP constants and how to define them with the define() method and the const keyword.

A constant is merely a term that has a single value associated with it. A constant’s value cannot be changed throughout the execution of a PHP script, as its name implies.

The define() method is used to create a constant. The first argument to the define() function is the name of the constant, and the second argument is the value of the constant. Consider the following example:

<?php 

define('WIDTH','1140px');
echo WIDTH;

Constant names are always written in uppercase. Unlike variables, the name of a constant does not begin with the dollar symbol ($).

Constant names are case-sensitive by default. It signifies that Breadth and width are not the same thing.

A constant in PHP 5 can hold a simple value such as a number, a string, or a boolean value. A constant can now hold an array since PHP 7.0. Consider the following scenario:

<?php

define( 'ORIGIN', [0, 0] );

Constants, like superglobal variables, can be accessed from anywhere in the script.

Const Keyword in PHP

The const keyword in PHP gives you another way to specify a constant. The syntax is as follows:

const CONSTANT_NAME = value;

The constant name is defined following the const keyword in this syntax. You use the assignment operator (=) and the constant value to assign a value to a constant. A number, a string, or an array can be used as a scalar constant value.

The SALES TAX constant is defined in the following example with the const keyword:

<?php

const SALES_TAX = 0.085;

$gross_price = 100;

$net_price = $gross_price * (1 + SALES_TAX);

echo $net_price; // 108.5

The const keyword is used in the following example to define an RGB constant that holds an array:

<?php

const RGB = ['red', 'green', 'blue'];

const vs define in Python

The define() function is a function, but the const language construct is a language construct.

The define() function declares a constant at runtime, whereas the const keyword declares a constant at compile time.

To put it another way, you can conditionally define a constant using the define() method, as shown below:

<?php

if(condition) 
{
    define('WIDTH', '1140px');
}

The const keyword, on the other hand, cannot be used to define a constant in this manner. The syntax of the following code, for example, is incorrect:

<?php 

if(condition) 
{
   const WIDTH = '1140px';
}

Second, you can use the define() method to create a constant with a name that is derived from an expression. For example, the following defines three constants with the values 1, 2, and 3: OPTION 1, OPTION 2, and OPTION 3.

<?php

define('PREFIX', 'OPTION');


define(PREFIX . '_1', 1);
define(PREFIX . '_2', 2);
define(PREFIX . '_3', 3);

The const keyword, on the other hand, cannot be used to specify a constant name derived from an expression.

You can use the const keyword to define constants to make the code more obvious unless you wish to specify a constant conditionally or with an expression.