diff -u b/includes/OpenIDConnectClientBase.class.php b/includes/OpenIDConnectClientBase.class.php
--- b/includes/OpenIDConnectClientBase.class.php
+++ b/includes/OpenIDConnectClientBase.class.php
@@ -209,2 +209,10 @@
   }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function endSession($id_token, $post_logout_redirect_url = '') {
+    // No need to do anything, but make the function have a body anyway
+    // so that it's callable by overriding methods.
+  }
 }
only in patch2:
unchanged:
--- a/includes/OpenIDConnectClientInterface.class.php
+++ b/includes/OpenIDConnectClientInterface.class.php
@@ -129,4 +129,17 @@ public function decodeIdToken($id_token);
    */
   public function retrieveUserInfo($access_token);
 
+  /**
+   * End User Session in IdP.
+   *
+   * @param string $id_token
+   *   A NON expired id_token (RECOMMENDED).
+   * @param string $post_logout_redirect_url
+   *   Logout url registered on the IdP.
+   *
+   * @see https://openid.net/specs/openid-connect-session-1_0-28.html
+   * @see https://openid.net/specs/openid-connect-session-1_0-28.html#RedirectionAfterLogout
+   */
+  public function endSession($id_token, $post_logout_redirect_url = '');
+
 }