php 70 lines · 9 steps

HTTP content negotiation in PHP

Parse an Accept header, rank its media types by quality, and pick the best match your server can offer.

Explained by highlit
Intermediate http content-negotiation parsing sorting pattern-matching
1<?php
2 
3namespace App\Http;
4 
5final class ContentNegotiator
6{
7 public function __construct(private array $supported = ['application/json', 'application/xml', 'text/html'])
8 {
9 }
10 
11 public function best(?string $acceptHeader): ?string
12 {
13 $candidates = $this->parse($acceptHeader ?? '*/*');
14 
15 foreach ($candidates as $candidate) {
16 foreach ($this->supported as $offered) {
17 if ($this->matches($candidate['type'], $offered)) {
18 return $offered;
19 }
20 }
21 }
22 
23 return null;
24 }
25 
26 private function parse(string $header): array
27 {
28 $accepted = [];
29 
30 foreach (explode(',', $header) as $index => $part) {
31 $segments = array_map('trim', explode(';', $part));
32 $type = strtolower(array_shift($segments));
33 
34 if ($type === '') {
35 continue;
36 }
37 
38 $quality = 1.0;
39 foreach ($segments as $segment) {
40 if (str_starts_with($segment, 'q=')) {
41 $quality = (float) substr($segment, 2);
42 }
43 }
44 
45 $accepted[] = ['type' => $type, 'q' => $quality, 'order' => $index];
46 }
47 
48 usort($accepted, static function (array $a, array $b): int {
49 return $b['q'] <=> $a['q'] ?: $a['order'] <=> $b['order'];
50 });
51 
52 return array_values(array_filter($accepted, static fn (array $a): bool => $a['q'] > 0.0));
53 }
54 
55 private function matches(string $accepted, string $offered): bool
56 {
57 if ($accepted === '*/*') {
58 return true;
59 }
60 
61 [$acceptedType, $acceptedSub] = explode('/', $accepted) + [1 => '*'];
62 [$offeredType] = explode('/', $offered);
63 
64 if ($acceptedSub === '*') {
65 return $acceptedType === $offeredType;
66 }
67 
68 return $accepted === $offered;
69 }
70}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Content negotiation ranks the client's stated preferences against what the server actually supports.
  2. 2The q parameter and header order together define preference, so both must feed the sort.
  3. 3Wildcard media types like */* and type/* need explicit matching rules beyond string equality.

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