WordPress Plugin to Export Data

I'm developing my first WP plugin to extract data from a WordPress DB.

I want it to be possible to define the date range in order to limit what's gonna be extracted or extract it through invoices.

Can anyone have a look and tell me what's the problem or what I have to do?

That would be great, thanks!

/**
* Constructor
*/
function __construct() {
    if (isset($_GET['report'])) {

        $csv = $this->generate_csv();
        header("Pragma: public");
        header("Expires: 0");
        header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
        header("Cache-Control: private", false);
        header("Content-Type: application/octet-stream");
        header("Content-Disposition: attachment; filename=\"report.csv\";");
        header("Content-Transfer-Encoding: binary");

        echo $csv;
        exit;
    }

    // Add extra menu items for admins
    add_action('admin_menu', array($this, 'admin_menu'));

    // Create end-points
    add_filter('query_vars', array($this, 'query_vars'));
    add_action('parse_request', array($this, 'parse_request'));
}

/**
* Add extra menu items for admins
*/
function admin_menu() {
    add_menu_page('Extract Data', 'Extract Data', 'manage_options', 'export_data', array($this, 'export_data'));
}

/**
* Allow for custom query variables
*/
function query_vars($query_vars) {
    $query_vars[] = 'export_data';
    return $query_vars;
}

/**
* Parse the request
*/
function parse_request(&$wp) {
    if (array_key_exists('export_data', $wp->query_vars)) {
        $this->export_data();
        exit;
    }
}

/**
* Download report
*/
function export_data() {
    ob_start();?>

        <div class="wrap">

            <div id="icon-options-general" class="icon32"></div>
            <h2> WordPress Extract Data</h2>

            <p>
                <h3>Extract data throught defining the start/end date or through invoices</h3>

            </p>


            <div id="calendar_box">
                <h4>Search by date</h4>

                <form method="POST" action="" enctype="multipart/form-data">
                    Start Date:

                    <input type="text" name="date_picker1" id="date_picker1" />
                    <?php
                        echo '<img src="' . plugins_url( 'img/calendar.jpg', __FILE__ ) . '" />';
                    ?> 
                    <div id="calendar1" style="position: absolute; top: 40px;left: 5px; z-index:1;"></div>

                    End Date:

                    <input type="text" name="date_picker2" id="date_picker2" /> 
                    <?php
                        echo '<img src="' . plugins_url( 'img/calendar.jpg', __FILE__ ) . '" />';
                    ?>  
                    <div id="calendar2" style="position: absolute; top: 40px;left: 220px; z-index:1;"></div>

                    <br>                
                    <p class="submit">
                        <a href="?page=export_data&report=users"><input type="submit" class="button-primary" value="Export Report" /></a>
                    </p>
                </form>

            </div>

            <br>

            or

            <br>

            <div id="invoices_box">
                <h4>Search by invoice numbers</h4>
                <form method="POST" action="" enctype="multipart/form-data">
                    <input type="text" name="invoices" id="invoices" /> 

                    <br>
                    <p class="submit">
                        <input type="submit" class="button-primary" value="Export Report" />
                    </p>
                </form> 
            </div>



        </div>     

    <?php

    echo ob_get_clean();
}


/**
* Converting data to CSV
*/
function generate_csv() {
    global $wpdb;
    $csv_output = '';
    $table = 'wp_test_payments';
    $startdate = $_POST['date_picker1'];
    $finaldate = $_POST['date_picker2'];
    $invoices = $_POST['invoices'];
    $rowq1 = $wpdb->num_rows($MyQuery1);
    $rowq2 = $wpdb->num_rows($MyQuery2);


    /*
    *Build your Query
    */
    $MyQuery1 = $wpdb->get_results($wpdb->prepare('SELECT * FROM ' . $table . ' WHERE date BETWEEN ' . $startdate . ' AND '.$finaldate.' '));
    $MyQuery2 = $wpdb->get_results($wpdb->prepare('SELECT * FROM ' . $table . ' WHERE payment_id = ' . $invoices . ' '));

    $i = 0;

    if ( $rowq1 = 0 AND $rowq2 = 0) {

        $Error = $wpdb->print_error();

        die("You have to select one of the variables.");
    }

    if ($rowq1 > 0) {
        while ($row = $wpdb->get_results($MyQuery1)) {
            $csv_output = $csv_output . $row['Field'] . ",";
            $i++;
        }
    }

    if ($rowq2 > 0) {
        while ($row = $wpdb->get_results($MyQuery2)) {
            $csv_output = $csv_output . $row['Field'] . ",";
            $i++;
        }
    }

    $csv_output .= "\n";

    $values = ($rowq1 = 0 XOR $rowq2 = 0);
    while ($rowr = $wpdb->get_results($values)) {
        for ($j = 0; $j < $i; $j++) {
            $csv_output .= $rowr[$j] . ",";
        }
        $csv_output .= "\n";
    }

    return $csv_output;
}

via Chebli Mohamed