allow to get project id from ServiceAccountCredentials

composer.json

@@ -1,9 +1,9 @@
 {
   "name": "firebase-php/google-auth",
   "type": "library",
-  "description": "Google Auth Library for PHP",
-  "keywords": ["google", "oauth2", "authentication"],
-  "homepage": "http://github.com/google/google-auth-library-php",
+  "description": "A fork of Google Auth Library used in Firebase PHP project",
+  "keywords": ["google", "oauth2", "authentication", "firebase-php"],
+  "homepage": "http://github.com/firebase-php/google-auth",
   "license": "Apache-2.0",
   "require": {
     "php": ">=5.4",

src/Credentials/ServiceAccountCredentials.php

@@ -66,6 +66,11 @@ class ServiceAccountCredentials extends CredentialsLoader implements SignBlobInt
      */
     protected $auth;
 
+    /**
+     * @var string|null
+     */
+    private $projectId;
+
     /**
      * Create a new ServiceAccountCredentials.
      *
@@ -98,6 +103,7 @@ public function __construct(
             throw new \InvalidArgumentException(
                 'json key is missing the private_key field');
         }
+        $this->projectId = isset($jsonKey['project_id']) ? $jsonKey['project_id'] : null;
         $this->auth = new OAuth2([
             'audience' => self::TOKEN_CREDENTIAL_URI,
             'issuer' => $jsonKey['client_email'],
@@ -195,4 +201,9 @@ public function getClientName(callable $httpHandler = null)
     {
         return $this->auth->getIssuer();
     }
+
+    public function getProjectId()
+    {
+        return $this->projectId;
+    }
 }