Yesterday, a question tagged #psr7 on Twitter caught my eye:

#psr7 Request::getHeader($name) return array of single string instead of strings in #Slim3? cc: @codeguy pic.twitter.com/ifA9hCKAPs @feryardiant (tweet)

The image linked provides the following details:

When I call $request->getHeader('Accept') for example, I was expected that I'll get something like this: Array( [0] => text/html, [1] => application/xhtml+xml, [2] => application/xml, ) but, in reallity I got this: Array( [0] => text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 ) Is it 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.

Headers in PSR-7

When creating the PSR-7 specification, we had to juggle a fair number of details from the various HTTP specifications. Headers are one area that is particularly difficult, due to the flexibility and ambiguity in the specification.

The root of the ambiguity is that headers are allowed to have multiple values. Headers may have multiple values, but it's up to the specification for any given header.

Additionally how multiple values are represented is up to the given header. The HTTP specifications allow using multiple invocations for the same header:

X-Foo-Bar: baz X-Foo-Bar: bat

The above would mean that the X-Foo-Bar header has two values, baz and bat . Assuming the header allows multiple values at all; if it doesn't, then it has a single value, and the last representation wins ( bat , if you're paying attention).

The other way to represent multiple values is using a separator. The specifications indicate that if you want to have multiple values in the same header line, you should use a comma ( , ) as a separator. However, you may use any other separator you want. The SetCookie header is a prime example of a header allowing multiple values that uses a completely different separator (semicolon)!

So, to summarize:

A header may or may not allow multiple values.

Headers may be emitted more than once. If a header allows multiple values, then its value is the aggregate of each representation. If the header only allows one value, the last representation is the canonical value for that header.

Headers may use a separator character in a single line in order to separate multiple values. That character is suggested to be a comma, but it can vary per-header.

The other big ambiguity in the specification is that the specification is extensible, and specifically allows for custom headers.

This means that any general-purpose code representing HTTP, such as PSR-7, cannot possibly know the entire ruleset governing all possible HTTP messages, as it cannot know all potential header types, including whether they allow multiple values or not.

With these two facts in mind — headers may have multiple values, and custom headers are allowed — we made the following decisions with PSR-7:

All headers are collections

All headers are assumed to have multiple values. This gives consistency of usage, and puts the onus of knowing the semantics of any given header to the consumer.

For that reason, the most basic access for a given header, getHeader($name) , returns an array. That array can have the following values:

It can be empty; this means the header was not, or will not be, present in the representation.

A single string value.

More than one string value.

Naive Concatenation

Since the majority of headers only allow single values, and since most existing libraries that parse headers only accept strings, we provided another method, getHeaderLine($name) . This method guarantees return of a string:

If the header has no values, the string will be empty.

Otherwise, it concatenates the values using a comma.

We chose not to provide an argument indicating the separator to use, as the specification only indicates commas as separators, but also to reduce complexity of implementations. If you want to use a different separator, you can do so yourself using implode($separator, $message->getHeader($name)) .

No Parsing

Because separator characters vary per-header, and because different headers have different specifications regarding how to interpret the data, and because the specification allows custom headers we cannot code for in a general-purpose library, we decided that PSR-7 implementations must not parse header values provided to them.

Practically this has two effects:

For incoming requests, even if a header allows multiple comma-separated values, implementations must leave them intact. This ensures no data-loss.

For complex values, you must pass them to a parser to decompose and interpret them.

The rule also has another motivation: to provide a semantic for emitting headers with multiple values as either a single line or as multiple lines. If all values are concatenated in a single line, the client or server can assume that the message should be sent or was received with the header as a single line, while an array of multiple lines would indicate multiple header lines. This allows the consumer to decide how the header should be represented!

Ramifications

The path we chose has some interesting ramifications. First, we ended up with a highly consistent API. There's no ambiguity in terms of what data types I can expect when I call getHeader() or getHeaderLine() . Second, I can be assured that there has been no data loss once I have the results of one of those operations; no process has attempted to parse the value and potentially alter it.

The flip side is the Twitter comment from earlier. Let's look at that again.

Breaking it Down

Let's revisit what the author received from a getHeader('Accept') call:

Array( [0] => text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 )

The Accept header allows multiple values, but expects them as a single comma-concatenated string. Contrary to what the author expected, the above represents the following values:

[ 'text/html', 'application/xhtml+xml', 'application/xml;q=0.9', 'image/web', '*/*;q=0.8', ]

Note that the values include the ;q=* notations inline! The Accept header separates values with commas, but each value can have additional key/value attributes separated by semicolons as well.

Why isn't the above what we get from getHeader() ? It goes back to the last rule I mentioned regarding PSR-7 header treatment: no parsing. The Accept header specification indicates that multiple values should be on the same line, separated by commas, and that's precisely how browsers send it to the server; PSR-7 takes the line as-is and sets it as the sole value in the array.

Recommendations

The above 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.

What does the value indicate?

The client prefers text/html , application/xhtml+xml , and image/webp representations when possible; if any of those three are available, they are preferred in that order

, , and representations when possible; if any of those three are available, they are preferred in that order If none of the above are available, the next representation preferred is application/xml .

. Any other representation may be returned otherwise.

How do I know this? By reading the Accept header specification. Which is ridiculously complex. And for which a number of libraries are already written, which can accept the Accept header value, parse it, and return the priority queue for you. PSR-7 acts as the data source for such libraries, but does no parsing itself.

Fin

Hopefully, this post has demystified how PSR-7 represents and handles HTTP headers. PSR-7 was designed to mirror the extensibility of the HTTP specifications, provide consistency of usage, and data integrity.

One specific recommendation we made in the metadocument was that any processing of headers be delegated to dedicated libraries. I'm hoping to see more of these spring up as we see PSR-7 adoption ramp up.