Got rid of mystery $identifier class property, moved it to the getIdentifier method

src/Grant/ClientCredentialsGrant.php

@@ -19,13 +19,6 @@ use Psr\Http\Message\ServerRequestInterface;
  */
 class ClientCredentialsGrant extends AbstractGrant
 {
-    /**
-     * Grant identifier
-     *
-     * @var string
-     */
-    protected $identifier = 'client_credentials';
-
     /**
      * @inheritdoc
      */
@@ -47,4 +40,12 @@ class ClientCredentialsGrant extends AbstractGrant
 
         return $responseType;
     }
+
+    /**
+     * @inheritdoc
+     */
+    public function getIdentifier()
+    {
+        return 'client_credentials';
+    }
 }