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.

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Intermediate http content-negotiation parsing sorting pattern-matching

1<?php
2 
3namespace App\Http;
4 
5final class ContentNegotiator
6{
  public function __construct(private array $supported = ['application/json', 'application/xml', 'text/html'])
  {
  }
10 
  public function best(?string $acceptHeader): ?string
  {
      $candidates = $this->parse($acceptHeader ?? '*/*');
14 
      foreach ($candidates as $candidate) {
          foreach ($this->supported as $offered) {
              if ($this->matches($candidate['type'], $offered)) {
                  return $offered;
              }
          }
      }
22 
      return null;
  }
25 
  private function parse(string $header): array
  {
      $accepted = [];
29 
      foreach (explode(',', $header) as $index => $part) {
          $segments = array_map('trim', explode(';', $part));
          $type = strtolower(array_shift($segments));
33 
          if ($type === '') {
              continue;
          }
37 
          $quality = 1.0;
          foreach ($segments as $segment) {
              if (str_starts_with($segment, 'q=')) {
                  $quality = (float) substr($segment, 2);
              }
          }
44 
          $accepted[] = ['type' => $type, 'q' => $quality, 'order' => $index];
      }
47 
      usort($accepted, static function (array $a, array $b): int {
          return $b['q'] <=> $a['q'] ?: $a['order'] <=> $b['order'];
      });
51 
      return array_values(array_filter($accepted, static fn (array $a): bool => $a['q'] > 0.0));
  }
54 
  private function matches(string $accepted, string $offered): bool
  {
      if ($accepted === '*/*') {
          return true;
      }
60 
      [$acceptedType, $acceptedSub] = explode('/', $accepted) + [1 => '*'];
      [$offeredType] = explode('/', $offered);
63 
      if ($acceptedSub === '*') {
          return $acceptedType === $offeredType;
      }
67 
      return $accepted === $offered;
  }
70}

01 / 01

STEP 01

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Three takeaways

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

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