define()

define() is a PHP function, that introduces constant definitions.

define() is a function, and may be called anywhere. It is slower than const, as it is only executed at runtime. define() doesn’t allow class constants creation.

define() used to create case-insensitive constant, but this was abandoned in PHP 7.3.

<?php
    define('HELLO', 'Hello');

    print HELLO . " world\n";
?>

Documentation