The const Keyword

const is the simplest way to create a constant.

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<?php
const APP_NAME = "Task Manager";
const MAX_ITEMS = 50;

echo APP_NAME . " allows " . MAX_ITEMS . " items.";

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Task Manager allows 50 items.

Constants can hold arrays

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<?php
const ROLES = ["admin", "editor", "viewer"];

echo ROLES[1];

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editor

Constants can use other constants

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<?php
const SECONDS_PER_MINUTE = 60;
const SECONDS_PER_HOUR = SECONDS_PER_MINUTE * 60;

echo SECONDS_PER_HOUR;

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3600

The value must be something PHP can work out before the script runs, so a call to a normal function (such as date()) is not allowed in a const.

const inside classes

Class constants use const too. This is where you will use it most.

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<?php
class Order
{
    const STATUS_PAID = "paid";
    const STATUS_PENDING = "pending";
}

echo Order::STATUS_PAID;

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paid

From PHP 8.3 you can add a type to a class constant, for example const string STATUS_PAID = "paid";. On older versions, that is a parse error.

Rules of const

• It must be written at the top level of the file or inside a class. It cannot sit inside an if or a loop.
• The name is fixed when the file is compiled.

When you need a constant with a computed name or inside an if, use define() instead.