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Check if a remote URL is online or down.

PHP March 14, 2018


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


(Required) (String) The URL to check
Default: None

(Optional) (Boolean) Whether to bypass the cache
Default: true

(Optional) (Boolean) Allow actually calling the requested URL to check availability
Default: true

Parameter Notes

When false, $allow_remote_request will only check if the resource has been unavailable in the last 10 minutes. If the resource was unavailable, it will return false, otherwise it will return true (even if the resource is not available because it doesn’t actually check). This optimizes some calls because if the resource is available, it won’t need to be accessed twice. To use this properly, the resource must handle setting the “nebula_site_available_” transient manually.

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<?php if ( nebula()->is_available('') ){
    //Do something with the URL (like wp_remote_get).
} ?>

Demo is currently offline. is currently offline.
/r/Syracuse is currently offline. (a randomly generated domain) exists!

Additional Notes

This function caches the hostname (not the full URL) for 10 minutes. So whether the hostname is available or times out, that will be cached for 10 minutes unless the $allow_cache parameter is set to false. This means if a remote server is down, there will be a 5-10 second timeout (typically) once per hostname within 10 minutes.

Nebula’s remote_get() function builds this in so no manual transient needs to be maintained.

Source File

Located in /libs/Utilities/Utilities.php on line 859.

1 Hook

Find these filters and actions in the source code below to hook into them. Use do_action() and add_filter() in your functions file or plugin.

Need a new filter hook? Request one here.

This function has no action hooks available. Request one?

        public function is_available($url=null, $allow_cache=true, $allow_remote_request=true){
            $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 ){
                trigger_error('Error: Requested URL is either empty or missing acceptable protocol.', E_USER_ERROR);
                return false;

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

            //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*20); //Re-up the transient with a 15 minute expiration
                    return true; //This hostname has worked within the last 20 minutes

                set_transient('nebula_site_available_' . $hostname, 'Unavailable', MINUTE_IN_SECONDS*10); //10 minute expiration
                return false; //This hostname has not worked within the last 10 minutes

            //Make an actual request to the URL if: the transient was empty or specifically requested a non-cached response, and specifically allowing a lookup
            if ( $allow_remote_request ){
                $response = wp_remote_head($url); //Only get the head data for slight speed improvement

                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
                    return true;

            if ( !$allow_remote_request ){
                return true; //Resource may not actually be available, but was asked specifically not to check.

            set_transient('nebula_site_available_' . $hostname, 'Unavailable', MINUTE_IN_SECONDS*10); //10 minute expiration
            return false;


To override this PHP function, use this hook in your child theme or plugin ("my_custom" can be changed):

add_filter('pre_nebula_is_available', 'my_custom_is_available', 10, 4); //The last integer must be 1 more than the actual parameters
function my_custom_is_available($null, $url, $allow_cache, $allow_remote_request){ //$null is required, but can be ignored
    //Write your own code here

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

You can completely disable this PHP function with a single line actions:

 add_filter('pre_nebula_is_available', '__return_false');