You cannot call a PHP function that resides on the server by just clicking on a div that exists on the client browser.
You need to trigger a Javascript event (using e.g. jQuery), that will call a function on the server (e.g. through AJAX), that after checking the parameters are correct and the user has the right of calling it will do what you seek.
There are ready-made frameworks that would allow you to do that.
Otherwise (after including jQuery in your HTML page) you can do something like,
<div class="cross" id="deleteUserButton" data-user="nickname">X</div>
<script type="text/javascript">
$('#deleteUserButton').on('click', function() {
let nick = $(this).attr('data-user');
$.post('/services/delete.php',
{
cmd: 'delete',
user: nick
}).then( reply => {
if (reply.status === 'OK') {
alert("User deleted");
}
});
<?php
$cmd = $_POST['cmd'];
switch($cmd) {
case 'delete':
$user = $_POST['user'];
if (deleteUser($user)) {
$reply = [ 'status' => 'OK' ];
} else {
$reply = [ 'status' => 'failure', 'message' => 'Doh!' ];
}
break;
...
header('Content-Type: application/json;charset=UTF-8');
print json_encode($reply);
exit();