is_available()

nebula()->is_available($url, $nocache, $lookup_only, $args)

        public function is_available($url=null, $allow_cache=true, $allow_remote_request=true, $args=array()){
            $override = apply_filters('pre_nebula_is_available', null, $url, $allow_cache, $allow_remote_request);
            if ( isset($override) ){return $override;}

            //Make sure the URL is valid
            if ( empty($url) || strpos($url, 'http') !== 0 ){ //@todo "Nebula" 0: Update strpos() to str_contains() in PHP8
                trigger_error('Error: Requested URL is either empty or missing acceptable protocol.', E_USER_ERROR);
                return false;
            }

            $timer_name = $this->timer('Is Available (' . $url . ')', 'start', 'Is Available');
            $hostname = str_replace('.', '_', $this->url_components('hostname', $url)); //The hostname label for transients

            if ( $this->is_transients_enabled() ){
                //Check transient first
                $site_available_buffer = get_transient('nebula_site_available_' . $hostname);
                if ( !empty($site_available_buffer) && $allow_cache ){ //If this hostname was found in a transient and specifically allowing a cached response.
                    if ( $site_available_buffer === 'Available' ){
                        set_transient('nebula_site_available_' . $hostname, 'Available', MINUTE_IN_SECONDS*30); //Re-up the transient with a 30 minute expiration
                        $this->timer($timer_name, 'end');
                        return true; //This hostname has worked within the last 30 minutes
                    }

                    set_transient('nebula_site_available_' . $hostname, 'Unavailable', MINUTE_IN_SECONDS*15); //15 minute expiration
                    $this->timer($timer_name, 'end');
                    return false; //This hostname has not worked within the last 15 minutes
                }
            } else {
                $allow_remote_request = false; //If transients are being suspended, don't pre-check if URLs are available
            }

            //Make an actual request to the URL if: the transient was empty or specifically requested a non-cached response, and specifically allowing a lookup
            //Ex: remote_get() prevents this from running the actual request to avoid multiple requests. It handles the transient itself, so just looking up if a request has failed previously.
            if ( $allow_remote_request ){ //If we are actively allowed to make the request to check if the endpoint is actually available
                //Combine default args with passed args. Args docs: https://developer.wordpress.org/reference/classes/WP_Http/request/
                $all_args = array_merge(array(
                    'redirection' => 5, //Follow 5 redirects before quitting
                ), $args);

                //Only get the head data for slight speed improvement.
                $response = wp_remote_head($url, $all_args);
                if ( !is_wp_error($response) && $response['response']['code'] === 200 ){ //If the remote request was successful
                    set_transient('nebula_site_available_' . $hostname, 'Available', MINUTE_IN_SECONDS*20); //20 minute expiration
                    $this->timer($timer_name, 'end');
                    return true;
                }
            }

            if ( !$allow_remote_request ){ //Otherwise, do not actively check and just report that the site was not reportedly down prior
                $this->timer($timer_name, 'end');
                return true; //Resource may not actually be available, but was asked specifically not to check.
            }

            //Finally, if none of the previous checks were true, the site must be unavailable
            set_transient('nebula_site_available_' . $hostname, 'Unavailable', MINUTE_IN_SECONDS*10); //10 minute expiration
            $this->timer($timer_name, 'end');
            return false;
        }

add_filter('pre_nebula_is_available', 'my_custom_is_available', 10, 5); //The last integer must be 1 more than the actual parameters
function my_custom_is_available($null, $url, $allow_cache, $allow_remote_request, $args){ //$null is required, but can be ignored
    //Write your own code here

    return true; //Return true to prevent the original function from running afterwords
}

 add_filter('pre_nebula_is_available', '__return_false');