Constants

Constants in PHP are used to store a value and they cannot be changed or undefined afterwards during the execution of the script.

<?php

define('FOO', 'value');

Constant name start with a letter or underscore followed by any alfanumeric characters A-Z, a-z.

Scope of the constants is global. They can be used anywhere in your PHP script.

There are two ways to define constants in PHP:

• define('FOO', 'value')
• const BAR = 'value'

Each way has some benefits and drawbacks.

Contants defined with define

• define() is a function
• define() defines constants at run time
• slower compared to const

Constants defined with const

• const is a language construct
• const defines constants at compile time
• faster compared to function define()
• Constants cannot be declared using const inside functions, loops, if statements or try/catch blocks

<?php

const FOO = 'Hello';

// Outputs "Hello"
echo FOO;

const BAR = FOO.' world';

Array constants

Constants can be also arrays:

<?php

const PLANETS = ['Mercury', 'Venus', 'Earth', 'Mars', 'Jupiter', 'Saturn', 'Uranus', 'Neptune'];

// Outputs "Earth"
echo PLANETS[2];

Expressions

Constants can be also defined as expressions:

<?php

const FOO = 2 * 3;

// Outputs 6
echo FOO;

define('BAR', 3 * 4);

// Outputs 12
echo BAR;

Magic constants

Magic constants are special constants that are provided by PHP and have value depending where they are used.

Constants in PHP are resolved at runtime (when the script is parsed by PHP). Magical constants are resolved at compile time.

All magic constants are described below: