Matthew Weier O'Phinney has a new post to his site talking about PSR-7 and HTTP headers and how they (headers) are handled in the structure of this PHP-FIG specification.

Yesterday, a question tagged #psr7 on Twitter caught my eye: "When I call $request->getHeader('Accept') for example, I was expected that I'll get [an array] but, in reality I got [a string]. Is this correct?" In this post, I'll explain why the behavior observed is correct, as well as shed a light on a few details of header handling in PSR-7.

He talks about the planning that went into PSR-7 and how they had to work around some of the "flexibility" (quirks) in the HTTP specification. This was especially true when it came to repeated headers. He also walks through their thoughts on multiple header handling and that custom header values are allowed. Because of these two things, they decided to treat all headers as collections and, despite there being separators already in the values. Instead they collected headers of the same types into these collections, some containing only one value while others could contain multiple. Back to the question - this explains why the "Accept" header they desired was still in its comma-separated form and not split into the array they expected.

The [...] example provides another good lesson: Complex values should have dedicated parsers. PSR-7 literally only deals with the low-level details of an HTTP message, and provides no interpretation of it. Some header values, such as the Accept header, require dedicated parsers to make sense of the value.