This html code was generated of my php code, but I copied this out of the HTML code of the browser.
The console.log prints out undefined. I don't know why. It's probably a really dumb mistake, like always. Thank you for your help.
$('.show').click(function() {
var id = $(this).attr('id');
var div = $("div#" + id);
console.log(div.html());
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<h3>Heading <a href='#' class='show' id='2962'>+</a></h3>
<div class='.slidetoggle' id='2962' style='display: none;'>
<p>Test!</p>
</div>