WordPress external content loading

/**
 * Prints a specific part of the site when it's requested
 *
 * Listens to GET requests containing the "including_template_part" $_GET key and the "load_part"
 * $_GET key. If they're both set and the request comes from the same IP, the existence of a function called
 * "render_{$_GET['load_part']}" is checked($_GET['load_part'] gets all "-" replaced with "_").
 * If that function exists, it's called with or without parameters, depending on the value of $_GET['params'].
 */
function print_requested_template_part() {
    // Respond only to requests from the same address...
    if ( $_SERVER['REQUEST_METHOD'] == 'GET' && isset( $_GET['including_template_part'] ) && isset( $_GET['load_part'] ) && $_GET['load_part'] != '' && $_SERVER['REMOTE_ADDR'] == $_SERVER['SERVER_ADDR'] ) {
        $part = $_GET['load_part'];
        $func = 'render_' . str_replace( '-', '_', $part ); // if you have declared a function called "render_footer_include", then "?load_part=footer_include"
        if ( function_exists( $func ) ) {
            // Allow for passing parameters to the function
            if ( isset( $_GET['params'] ) ) {
                $params = $_GET['params'];
                $params = ( strpos( $params, ',' ) !== false ) ? explode( ',', $params ) : array( $params );
                call_user_func_array( $func, $params );
            } else {
                call_user_func( $func );
            }
        }
        exit; // if we don't exit here, a whole page will be printed => bad! it's better to have empty footer than a footer with the whole main site...
    }
}
add_action( 'init', 'print_requested_template_part', 1 );

/**
 * Retrieves the requested part of the main site
 *
 * Okay, well basically can be from any site, depending on the $url param, as long as it has the proper function that will display the requested content
 *
 * @param $url - the url of the site
 * @param $key - part of the name of the function that will display the content
 * @param $add_qs - any additional query string that will be appended, use "&params=param1,param2,param3" to pass "param1", "param2" and "param3"
 * to the loading function
 */
function get_main_site_part( $url, $key, $add_qs = '' ) {

    // cache the result, so we don't make a request with each page load
    $cache_key = md5( "main_site_{$key}" );
    $cache_lifetime = 300; // 300 seconds = 5 minutes. Increase if necessary

    // just to make sure - try to remove the trailing slash in the $url
    $url = add_query_arg( array( 'including_template_part' => '1', 'load_part' => $key ), $url ) . $add_qs;

    if ( ( $main_site_html = get_transient( $cache_key ) ) === false ) {
        $main_site_html = wp_remote_get($uri);
        if ( is_a( $main_site_html, 'WP_Error' ) ) {
            // You can have some kind of error handling here
            // For instance you can try again
            return;
        }
        // Update the transient
        set_transient( $cache_key, $main_site_html, $cache_lifetime );
    }

    // Either freshly downloaded, or comes from a valid transient
    return $main_site_html;
}