add_hreflang_attribute

/**
* ADD HREFLANG ATTRIBUTES
* Automatically append hreflang with network URL, language, and permalink to all pages
*/

function add_hreflang_attribute() {
   $site_url = network_site_url(); // base URL
   $alt_langs = array( '', 'au', 'uk' ); // two-letter language code
   $page_path = substr(get_permalink(), strlen(home_url('/'))); // path of page after base URL

   if (!( is_singular( 'locations' ) ) ) {

       if ( is_post_type_archive('locations') ) {

           echo '<link rel="alternate" href="' . $site_url . $page_path . '" hreflang="x-default" />';

           // loop through the alternative languages, and get the appropriate hreflang tag for each that exists
           foreach ($alt_langs as $lang) {
               $updated_url_lang_path = $site_url . $lang . '/locations/';
               $url_headers = @get_headers($updated_url_lang_path);
               if($url_headers && strpos( $url_headers[0], '200')) {
                   if ($lang == 'uk') {
                       echo '<link rel="alternate" href="' . $updated_url_lang_path . '" hreflang="en-gb" />'. PHP_EOL;
                   } else {
                       echo '<link rel="alternate" href="' . $updated_url_lang_path . '" hreflang="en-' . $lang . '" />'. PHP_EOL;
                   }
               }
           }

       } else {

           // loop through the alternative languages, and get the appropriate hreflang tag for each that exists
           foreach ($alt_langs as $lang) {
               $updated_url_lang_path = $site_url . $lang . '/' . $page_path;
               $url_headers = @get_headers($updated_url_lang_path);
               if($url_headers && strpos( $url_headers[0], '200')) {
                   if ($lang == 'uk') {
                       echo '<link rel="alternate" href="' . $updated_url_lang_path . '" hreflang="en-gb" />'. PHP_EOL;
                   } elseif ($lang == '') {

                   }
                   else {
                       echo '<link rel="alternate" href="' . $updated_url_lang_path . '" hreflang="en-' . $lang . '" />'. PHP_EOL;
                   }
               }
           }

           // set primary as x-default
           echo '<link rel="alternate" href="' . $site_url . $page_path . '" hreflang="x-default" />';

       }

   }

}
add_action('wp_head', 'add_hreflang_attribute', 1);