While not specific to PHP, the HTTP specification that defines how web applications talk has gotten a major update in its latest version. In his latest post Evert Pot summarizes some of these changes and how they'll impact the work you're doing.
The IETF just published several new RFCs that update HTTP/1.1 [...] These documents make the original specification for HTTP/1.1 obsolete. As a HTTP geek, this is a big deal.
These new RFCs include definitions of standards around message syntax and routing, conditional requests, authentication, the 308 status code and the forwarded HTTP extension. Evert gets into the details of some of the changes, pointing out the major changes first and places where ambiguity has been resolved. He also includes a list of other "interesting things that have changed" in these new specs including clarifications around dealing with unexpected whitespace, the removal of the default charset of ISO-8859-1 and that the 204, 404, 405, 414 and 501 status codes are now cacheable.