A service to send Webmentions to links found in HTML.

Webmention.php

<?php

namespace App\Services;

use App\Support\Str;
use Illuminate\Http\Client\Response;
use Illuminate\Support\Collection;
use Illuminate\Support\Facades\Config;
use Illuminate\Support\Facades\Http;

class Webmention
{
    public Collection $links;

    public function __construct(private string $html)
    {
        $this->links = new Collection();

        $this->discoverOutgoingLinks($html);
    }

    public function notify(string $source, string $target): Response|false
    {
        $endpoint = $this->discoverEndpoint($target);

        if (! $endpoint) {
            return false;
        }

        return Http::post($endpoint, [
            'source' => $source,
            'target' => $target,
        ]);
    }

    private function discoverOutgoingLinks(string $html): void
    {
        $url = Config::get('app.url');

        preg_match_all("/<a[^>]+href=.(https?:\/\/[^'\"]+)/i", $html, $matches);

        $this->links = $this->links
            ->merge(array_unique($matches[1]))
            ->reject(static fn (string $link): bool => Str::of($link)->startsWith($url));
    }

    private function discoverEndpoint(string $target): ?string
    {
        return $this->discoverEndpointInHeader($target) ?? $this->discoverEndpointInBody($target);
    }

    private function discoverEndpointInHeader(string $target): string|false
    {
        $response = Http::head($target);
        $headers = $response->headers();

        if (array_key_exists('Link', $headers)) {
            return false;
        }

        $linkHeader = is_array($headers['Link']) ? implode(", ", $headers['Link']) : $headers['Link'];

        if (preg_match(
            '~<((?:https?://)?[^>]+)>; rel="?(?:https?://webmention.org/?|webmention)"?~',
            $linkHeader,
            $match
        )) {
            return $match[1];
        }

        return false;
    }

    private function discoverEndpointInBody(string $target): string|false
    {
        $response = Http::get($target);

        $body = preg_replace('/<!--(.*)-->/Us', '', (string) $response->body());

        if (preg_match('/<(?:link|a)[ ]+href="([^"]*)"[ ]+rel="[^" ]* ?webmention ?[^" ]*"[ ]*\/?>/i', $body, $match)
            || preg_match('/<(?:link|a)[ ]+rel="[^" ]* ?webmention ?[^" ]*"[ ]+href="([^"]*)"[ ]*\/?>/i', $body, $match)
        ) {
            return $match[1];
        }

        return false;
    }
}