Fix undefined index error

Getting $op is handled at the top of the file, use the same variable
at the end of the file to avoid errors about an undefined index.

backend.php

@@ -161,6 +161,6 @@
 	}
 
 	header("Content-Type: text/json");
-	print Errors::to_json(Errors::E_UNKNOWN_METHOD, [ "info" => (isset($handler) ? get_class($handler) : "UNKNOWN:".$_REQUEST["op"]) . "->$method"]);
+	print Errors::to_json(Errors::E_UNKNOWN_METHOD, [ "info" => (isset($handler) ? get_class($handler) : "UNKNOWN:".$op) . "->$method"]);
 
 ?>