PHP Constants

A constant is a value that never changes while the program runs. Unlike variables, constants do not use a dollar sign and cannot be reassigned.

Defining with define()

The define() function creates a constant at runtime.

<?php
define("SITE_NAME", "My Website");
echo SITE_NAME; // My Website

Defining with const

The const keyword defines a constant at compile time, often used at the top level or inside classes.

<?php
const MAX_USERS = 100;
echo MAX_USERS; // 100

Rules for constants

• No dollar sign in the name.
• By convention, names are written in uppercase.
• The value cannot change once set.

Magic constants

PHP provides special predefined constants that change depending on where they are used.

<?php
echo __LINE__; // current line number
echo __FILE__; // full path of the file
echo __DIR__;  // directory of the file

These are useful for debugging and logging. Note they begin and end with two underscores.