There is a wonderful blog post from PayPal in which it is introduced a very easy way to set a payee in your express checkout flow, but sadly there are some open issues with that.
Here the same is provided using the PayPal PHP SDK.
Please remember to change some values in order to test it (every file has its own comment to tell which variable to change). Also, obviously, the credentials should be stored in a secure way when in a production environment.
You can find the complete code, as usual, on my github profile.