don't force using GCM API key for FCM

Author: Minishlink

src/WebPush.php

@@ -213,16 +213,18 @@ private function prepareAndSend(array $notifications)
             }
 
             // if GCM or FCM url
-            if (substr($endpoint, 0, Utils::safe_strlen(self::GCM_URL)) === self::GCM_URL
-                || substr($endpoint, 0, Utils::safe_strlen(self::FCM_URL)) === self::FCM_URL) {
+            $isGCM = substr($endpoint, 0, strlen(self::GCM_URL)) === self::GCM_URL;
+            if ($isGCM || substr($endpoint, 0, strlen(self::FCM_URL)) === self::FCM_URL) {
                 if (array_key_exists('GCM', $this->auth)) {
                     $headers['Authorization'] = 'key='.$this->auth['GCM'];
-                } else {
+                } elseif ($isGCM) {
+                    // FCM doesn't need an API key, it's optional if you're using VAPID
                     throw new \ErrorException('No GCM/FCM API Key specified.');
                 }
             }
-            // if VAPID
-            elseif (array_key_exists('VAPID', $this->auth)) {
+
+            // if VAPID (GCM doesn't support it but FCM does)
+            if (array_key_exists('VAPID', $this->auth) && !$isGCM) {
                 $vapid = $this->auth['VAPID'];
 
                 $audience = parse_url($endpoint, PHP_URL_SCHEME).'//'.parse_url($endpoint, PHP_URL_HOST);