[ticket/16955] Add stubs for cache classes and clean up

PHPBB3-16955

phpBB/phpbb/cache/driver/dummy.php

@@ -95,6 +95,14 @@ class dummy extends \phpbb\cache\driver\base
 		return false;
 	}
 
+	/**
+	 * {@inheritDoc}
+	 */
+	protected function _read(string $var)
+	{
+		return false;
+	}
+
 	/**
 	* {@inheritDoc}
 	*/