CakePHP 3.x: set variable from ajax call - php

Question

The goal is to set a variable (stored into AppController.php) whenever the user select a new value from a list.
nav-top.ctp
<?= $this->Form->create(false, ['id' => 'ajaxForm']) ?>
<?= __('Year') ?>
<?= AppController::year(); ?> <---- just for test
<select name="currentYear">
<option value="2020">2020</option>
<option value="2021" selected>2021</option>
</select>
<?= $this->Form->end(); ?>
...
<?php $this->append('script'); ?>
<script type="text/javascript">
$("#currentYear").on("change", function() {
let ajaxdata = $('#ajaxForm').serializeArray();
$.ajax({
type: "POST",
url: "app/setYear",
data: ajaxdata,
success: function(data) {
$(document.body).html(data);
},
error: function (xhr, status, error) {
$(document.body).html(error);
}
});
});
</script>
<?php $this->end(); ?>
AppController.php
class AppController extends Controller
{
private $_year = 2021;
...
public function year()
{
return new Date($this->_year . '-01-01');
}
public function setYear()
{
$this->_year = $this->request->getData('currentYear');
return $this->redirect($this->referer());
}
}
When I change the current item in the select the POST is correctly fired and I get the new value inside the setYear() method.
Then the page reloads, as expected. But all the queries that rely on the year() function, i.e.:
public function index()
{
$this->paginate = [
'contain' => [
'Customers',
'ProformaStates',
'ItemProformas' => ['ItemDeliveryNotes' => ['DeliveryNotes']]
],
'conditions' => [
'proforma_date >=' => AppController::year()
],
'order' => ['proforma_number' => 'desc']
];
$proformas = $this->paginate($this->Proformas);
$this->set(compact('proformas'));
}
still use the old value, like it has not changed.
Furthermore, if I enable the test line above (to printout in the reloaded page the actual value of year) I get the following error:
Using $this when not in object context