Superglobal scope of class static properties, at the risk of stating the obvious.

<?php
class Alfa {
    public static $superglobal = 'visible everywhere';
}

function demoGlobal( ) {
    echo 'Alfa::$superglobal '.Alfa::$superglobal;
}

demoGlobal();   // outputs Alfa::$superglobal visible everywhere
?>