I got this varible:
var things = $('#box li');
My problem is that the user can change the value in #box. So it contains x different li:s then my var things is not set anymore. How do I change this var? var things is in my index.php file. The place where the user changes the #box is in my change.php file, so I think It's a bit harder.
Probably I duplicate but I dont know what to search so please direct me if you can.
Use getElementsByTagName, which will return a live HTMLCollection, which you can select again:
var things = $('#box')[0].getElementsByTagName("li")
console.log($(things).toArray())
$('li').last().remove() //remove last li element
console.log($(things).toArray())<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="box">
<ul>
<li>a</li>
<li>b</li>
<li>c</li>
</ul>
</div>