PayPal Express Checkout on behalf of a client php

I've received permissions for Express Checkout from my client using the Permissions Service, getting token and token-secret and I can generate the X-PAYPAL-AUTHORIZATION header [contains AccessToken, OAuthSignature, CurrentOAuthTimestamp] as described 'http://ift.tt/1payxXm'

I don't understand how can I call the SetExpressCheckout API on behalf of my client. I don't see the X-PAYPAL-AUTHORIZATION header mentioned in the Express Checkout docs. http://ift.tt/2gM3yUo

via Chebli Mohamed