Using wordpress functions in class and change my code to OOP PHP

Thank you guys for all the suggestions and answers. I have fixed it, see my code below:

class advanza_form {
// Class properties defined using PHP Coding Guidelines and taking security measures: http://www.php.net/manual/en/language.oop5.visibility.php
public $site_id;
public $affiliate_id;
public $redirection;
protected $url;
private $response;
private $header; // For debugging
public $status_code; // For debugging

public function __construct() {
    $this->site_id = get_option('advanza_form_site_id');
    $this->affiliate_id = get_option('advanza_form_affiliate_id');
    $this->redirection = get_option('advanza_form_thankyou_page');
    $this->url = "https://advanzaservices.com/wp/singleform.php?site_id={$this->site_id}&affiliate_id={$this->affiliate_id}&thankyou={$this->redirection}";
    $this->response = wp_remote_get( $this->url );
    $this->header = wp_remote_retrieve_headers( $this->response ); // array of http header lines
    $this->status_code = wp_remote_retrieve_response_code( $this->response );
}

public function display() {
    try {
        if($this->status_code == 200) {
            $html = wp_remote_retrieve_body($this->response);
        } else {
            return $this->status_code;
        }
    } catch (Exception $e) {
        echo 'Caught exception: ', $e->getMessage(), "\n";
    }// end try/catch

    return $html;
}

public function debug() {
    $last_modified = wp_remote_retrieve_header( $this->response, 'last-modified' );
    echo '<h1>Debug information</h1>';
    echo '<p>';
    echo '<strong>Site ID:</strong> ' . $this->site_id . '<br>';
    echo '<strong>Affiliate ID:</strong> ' . $this->affiliate_id . '<br>';
    echo '<strong>Redirection:</strong> ' . $this->redirection . '<br>';
    echo '<strong>URL:</strong> ' . $this->url . '<br>';
    echo '</p>';
    echo '<h2>Technical details</h2>';
    echo '<p>';
    echo '<a href="https://en.wikipedia.org/wiki/List_of_HTTP_status_codes" target="_blank"><strong>HTTP status code:</strong></a> ' . $this->status_code . '<br>';
    echo '<strong>Headers:</strong> '; var_export($this->header);
    echo '</p>';
    // error: Invalid or no affiliate_id supplied
}
}

The shortcode

function advanza_form_shortcode() {
    // Display the Advanza form with a shortcode
    $form = new advanza_form;
    // For debugging
    // $form->debug();
    return $form->display();
}
add_shortcode( 'advanza-form', 'advanza_form_shortcode' );