Create Multi-Page Form in WordPress

add_shortcode('multipage_form_sc','multipage_form');
function multipage_form(){
global $wpdb;
  $this_page    =   $_SERVER['REQUEST_URI'];
  $page     =   $_POST['page'];
if ( $page == NULL ) {
echo '<form method="post" action="' . $this_page .'">
  <label for="first_name" id="first_name">First Name: </label>
<input type="text" name="first_name" id="first_name" />
<label for="last_name" id="last_name">Last Name: </label>
<input type="text" name="last_name" id="last_name" />
<label for="email" id="email">Email: </label>
<input type="text" name="email" id="email" />
<label for="phone" id="phone">Phone: </label>
<input type="text" name="phone" id="phone" />
<label for="first_name" id="first_name">Zip Code: </label>
<input type="text" name="zip_code" id="zip_code" />
<input type="hidden" value="1" name="page" />
<input type="submit" />
  </form>';
  }//End Page 1 of Form
elseif ( $page == 1 ) {
$first_name =   $_POST['first_name'];
  $last_name    =   $_POST['last_name'];
  $email        =   $_POST['email'];
  $phone        =   $_POST['phone'];
$zip_code   =   $_POST['zip_code'];
echo '<h3>You made it to the 2nd page!</h3>
<p>Here are your form inputs: </p>
  <p>First Name: ' . $first_name . '</p>
  <p>Last Name: ' . $last_name . '</p>
  <p>Email: ' . $email . '</p>
  <p>Phone: ' . $phone . '</p>
  <p>Zip Code: ' . $zip_code . '</p>';
  }//End Page 2 of Form
  };