<?php
    $variabele1 = 1;
    $variabele2 = 'Samenbrengen';
    $variabele3 = array();
    $variabele4 = null;
    var_dump(isset($variabele1, $variabele2, $variabele3, $variabele4));
    /* false */ //$variabele4 is null dus isset retourneert false
?>