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Constants can be defined using the const keyword, or by using the define()-function. While define() allows a constant to be defined to an arbitrary expression, the const keyword has restrictions. Once a constant is defined, it can never be changed or undefined.
<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // Emits an Error: Undefined constant "Constant"
// Prior to PHP 8.0.0, outputs "Constant" and issues a warning.
?>
https://www.php.net/manual/en/language.constants.syntax.php