Use PHP code to force Wordpress to redirect to a secure site

add-to-your-theme-header.php

/*
 * 2016 Jan 29
 *
 * For redirecting to SSL site, from within PHP.
 *
 * This is useful if:
 *  - Your site is running behind a load-balancer, which
 *    means you cannot directly infer HTTPS, AND
 *  - you don't have the ability to modify
 *    .htaccess on your hosted site.
 *
 * Add this to your theme header, or similar.
 * The code must run before any content is emitted,
 * because a redirect won't work after that.
 *
 */

function so_site_wants_ssl() {
  // get_site_url() is actual url used.
  // get_home_url() is url configured in the WP settings panel
  $my_scheme = substr(get_home_url(), 0, 6);
  return (strcasecmp($my_scheme, "https:") == 0) ;
}

function smarter_is_ssl_behind_lb() {
  return (is_ssl() &&
          isset($_SERVER['HTTP_X_FORWARDED_PROTO']) &&
          ($_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https'));
}

function so_maybe_redirect_to_ssl_site() {
  // force ssl if site is configured for SSL and if user is not using SSL
  if (so_site_wants_ssl()) {
    // site desires https access
    if (!smarter_is_ssl_behind_lb()) {
      // runtime thinks it is not being accessed via SSL
      // redirect to secure endpoint
      wp_redirect(get_home_url());
      exit;
    }
    $_SERVER['HTTPS']='on'; // make is_ssl() do the right thing
  }
}

so_maybe_redirect_to_ssl_site();