getting an error "Security Header is not valid" while making webiste payment pro api call. any help in this regard will be appreciated.

Never, ever, accept code from a insecure website to handle anything to do with real money.
You do not know who is giving you the code, you do not know what it does, you do not know that it places the monies correctly into the appropriate account, without passing the details to any third parties.

Only get such code from Paypal themselves - the scope for fraud otherwise is far too large. And remember, you personally could be liable for any monies lost if your action is seen to be negligent - which getting your code from a public forum would most certainly be!

They are pretty helpful - give them a try!