PHP Class auto run method - php

Question

I don't know if the title is really correct for what I'm asking, but here it goes. I'm trying to build a very basic pagination class, just for retrieving a range of items.
This is my class so far
<?php
Class Pagination {
private $_dbh;
private $_currentPage;
private $_totalPages;
private $_startLimit;
private $_totalItems;
private $_table = 'products';
private $_perPage = 8;
private $_allowedTables = ['products', 'cart', 'orders'];
public $_results = [];
public function __construct($dbh = null) {
$this->_dbh = ($dbh !== null) ? $dbh : null;
}
public function getResults() {
$this->_results = $this->_dbh->query(" SELECT * FROM $this->_table ORDER BY id DESC LIMIT $this->_startLimit, $this->_perPage ")->fetchAll();
}
public function setCurrentPage($currentPage = null) {
$this->_currentPage = ($currentPage !== null && is_int($currentPage)) ? $currentPage : 1;
}
public function setPerPage($perPage = null) {
$this->_perPage = ($perPage !== null && is_int($perPage)) ? $perPage : $this->_perPage;
}
public function setTable($table = null) {
$this->_table = ($table !== null && in_array($table, $this->_allowedTables)) ? $table : $this->_table;
}
private function totalItems() {
$this->_totalItems = $this->_dbh->query(" SELECT COUNT(id) AS total FROM $this->_table")->fetch()->total;
}
private function totalPages() {
$this->_totalPages = ceil($this->_totalItems / $this->_perPage);
}
private function startLimit() {
$this->_startLimit = ( $this->_currentPage - 1 ) * $this->_perPage;
}
public function getInfo() {
return array(
'table' => $this->_table,
'perPage' => $this->_perPage,
'currentPage' => $this->_currentPage,
'totalItems' => $this->_totalItems,
'totalPages' => $this->_totalPages,
'startLimit' => $this->_startLimit,
'query' => " SELECT * FROM $this->_table ORDER BY id DESC LIMIT $this->_startLimit, $this->_perPage "
);
}
}
?>
It is incomplete, but this is how I would like to call it
$pagination = new Pagination($dbh); // $dbh = PDO connection
$pagination->setCurrentPage(2);
$pagination->setTable('products'); // optional
$pagination->setPerPage(12); // optional
$products = $pagination->_results();
The problem is that all the variables that I define at the top of my class ( $_currentPage, $_totalPages, $_startLimit and $_totalItems ) are empty.
Even when I set them, they are still empty(as I expect) so I cannot calculate the $_totalPages, $_startLimit, $_totalItems or call the method getResults() which will query the DB for the items.
The first approach was to do everything in the __construct method and it worked, but I feel like it's not good to perform too many actions inside one method.
So my question is how can I modify my class so I can use it as I showed even without calling the setter methods ?
Plus, all the methods that calculate the number of pages, total items, etc.., would have to be called automatically from inside the class so they actually set the variables values, the way it is now they are only available but aren't run anywhere.
Thank you.