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

src/Grant/AbstractGrant.php

@@ -31,13 +31,6 @@ abstract class AbstractGrant implements GrantTypeInterface
 {
     const SCOPE_DELIMITER_STRING = ' ';
 
-    /**
-     * Grant identifier
-     *
-     * @var string
-     */
-    protected $identifier = '';
-
     /**
      * Grant responds with
      *
@@ -141,14 +134,6 @@ abstract class AbstractGrant implements GrantTypeInterface
         $this->refreshTokenTTL = $refreshTokenTTL;
     }
 
-    /**
-     * {@inheritdoc}
-     */
-    public function getIdentifier()
-    {
-        return $this->identifier;
-    }
-
     /**
      * {@inheritdoc}
      */
@@ -317,7 +302,7 @@ abstract class AbstractGrant implements GrantTypeInterface
     {
         return (
             isset($request->getParsedBody()['grant_type'])
-            && $request->getParsedBody()['grant_type'] === $this->identifier
+            && $request->getParsedBody()['grant_type'] === $this->getIdentifier()
         );
     }
 }

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';
+    }
 }

src/Grant/GrantTypeInterface.php

@@ -31,7 +31,7 @@ interface GrantTypeInterface
     public function setRefreshTokenTTL(\DateInterval $refreshTokenTTL);
 
     /**
-     * Return the identifier
+     * Return the grant identifier that can be used in matching up requests
      *
      * @return string
      */

src/Grant/PasswordGrant.php

@@ -24,13 +24,6 @@ use Psr\Http\Message\ServerRequestInterface;
  */
 class PasswordGrant extends AbstractGrant
 {
-    /**
-     * Grant identifier
-     *
-     * @var string
-     */
-    protected $identifier = 'password';
-
     /**
      * @var \League\OAuth2\Server\Repositories\UserRepositoryInterface
      */
@@ -109,4 +102,12 @@ class PasswordGrant extends AbstractGrant
 
         return $user;
     }
+
+    /**
+     * @inheritdoc
+     */
+    public function getIdentifier()
+    {
+        return 'password';
+    }
 }

src/Grant/RefreshTokenGrant.php

@@ -23,13 +23,6 @@ use Psr\Http\Message\ServerRequestInterface;
  */
 class RefreshTokenGrant extends AbstractGrant
 {
-    /**
-     * Grant identifier
-     *
-     * @var string
-     */
-    protected $identifier = 'refresh_token';
-
     /**
      * @var \League\OAuth2\Server\Repositories\RefreshTokenRepositoryInterface
      */
@@ -134,4 +127,12 @@ class RefreshTokenGrant extends AbstractGrant
 
         return $refreshTokenData;
     }
+
+    /**
+     * @inheritdoc
+     */
+    public function getIdentifier()
+    {
+        return 'refresh_token';
+    }
 }