Need Help in access token by John Hutchison

I am trying to use Shopify APP in PHP. I am following the authorization process specified in the Shopify Documentation.

I have recieved the Authorization Code in the callback url.

I am trying to exchange the Authorization Code for Access Token by posting a request on URL ( https://domain.myshopify.com/admin/oauth/access_token ) with all the parameters. But I am getting error of Bad Request (400).

I also performed HMAC and MD5 verification but it is failing there.

I think I am getting the wrong Authorization Code, please suggest what all issues can be there.

Thanks in advance