You can define constants with variable names (works also with constant values or variables or array values or class properties and so on - as long it's a valid constant name).
<?php
define("SOME_CONSTANT", "NEW_CONSTANT");
define(SOME_CONSTANT, "Some value");
echo SOME_CONSTANT; echo "<br>";
echo NEW_CONSTANT; ?>
Needless to say that you'll lose your IDE support for refactoring and highlighting completely for such cases.
No clue why someone would / could actually use this but i thought it's worth mentioning.