Please help me I am hitting Pardot Visitor Api but it is giving me error
You need to pass the user credentials in the HTTP Header, not in the URL.
Take a look at http://developer.pardot.com/#sample-code. Near the bottom of the PHP example, they show the authHeader should look something like:
Authorization: Pardot user_key=<YOUR_USER_KEY>,api_key=<API_KEY_FROM_AUTH_RESPONSE>