<?php

class SalesReport {
    // Data for report
    private $data = array();

    public function __construct($date_from, $date_to) {
        // TODO: connect to DB
        $this->data = array(
            $date_from => 100,
            $date_to => 200,
        );
    }
}

class PDFSalesReport extends SalesReport {

}
class XLSXSalesReport extends SalesReport {

}

$report_pdf = new PDFSalesReport('2018-01-01', '2018-01-02');