As Nicolas Grekas shows in this tutorial (posted as a gist), the functionality that PHP gives users to work with the raw HTTP requests is pretty limited. There's a few solutions he offers, but none of them allow for the full use of the HTTP options provided by the server.
HP offers comprehensive autoglobals to access external data sent with each request. These variables do not expose all the possibilities allowed by the HTTP protocol, but a controlled use can in practice minimize the impact of this limitation. Two problems are particularly troublesome: the lack of access to multi-valued keys without using a special syntax and complexity of the magic bracketed syntax. Until PHP natively provides another interface freed from these problems, a different interface in user space can circumvent them.
He gives some code examples of methods that can be used to parse strings like those found in cookies (or pulled in via php://stdin) including mentions of key normalization and accessing the values by literal keys. You can see a more full implementation here.