php function to generate signed_body sha256 HMAC in instagram Iphone app

I need a php function that can generate a signed_body as used by instagram. It must be able to produce the same strings the actual instagram app does and I can give examples/help if necessary.

the signed_body used by instagram application during account creation process seems to be a sha256 HMAC

I know you can generate an sha256 HMAC in php with the following code

hash_hmac('sha1', "Message", "Secret Key");

What I need to know from whomever takes this project is what is how to obtain the secret key that is used to generate the sha256 HMAC and what part of the data you send on signup is used as the "Message" to also generate the sha256 HMAC.

Assembly iPhone Java PHP

Project ID: #4320250

About the project