define()
define() is a PHP function that introduces global 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 the creation of class constants.
define() used to create case-insensitive constants, and this was abandoned in version 7.3.
define() cannot create class constants.
<?php
define('HELLO', 'Hello');
print HELLO . " world\n";
?>