I am trying using IPN simulator and my post back script always get INVALID response for the validation command post request. I also tried using PHP code in sandbox environment and it doesn't return anything and in production it always getting "INVALID URL" response.
Sounds like an issue with your script or how the data is being passed back for validation. The exact data which was received, must be sent back in the exact same order in which is was received and the character encoding must also match the original transmission.
