Fix FastCGI_Purger::purge_amp_version() for purge_method=unlink_files

Resolves rtCamp#396.

Author: stefanfisk

admin/class-fastcgi-purger.php

@@ -103,11 +103,13 @@ public function purge_url( $url, $feed = true ) {
 	 * @param string $url_base The base URL to purge.
 	 */
 	private function purge_amp_version( $url_base ) {
+		global $nginx_helper_admin;
+
 		$amp_url = sprintf( '%s/amp/', rtrim( $url_base, '/' ) );
 
 		$this->log( '- Purging AMP URL | ' . $amp_url );
 
-		if ( 'unlink_files' === $this->nginx_helper_admin->options['purge_method'] ) {
+		if ( 'unlink_files' === $nginx_helper_admin->options['purge_method'] ) {
 			$this->delete_cache_file_for( $amp_url );
 		} else {
 			$this->do_remote_get( $amp_url );