PHP Webhook Signature

Minimal, framework-agnostic HMAC-SHA256 webhook signature generation and verification with replay attack prevention.

Requirements

• PHP 8.2+

Installation

composer require philiprehberger/php-webhook-signature

Usage

Sender Side — Signing an Outgoing Payload

use PhilipRehberger\WebhookSignature\WebhookSignature;

$payload = json_encode(['event' => 'invoice.paid', 'id' => 42]);
$secret  = 'your-shared-webhook-secret';

$signatureHeader = WebhookSignature::generate($payload, $secret);
// "t=1700000000,v1=a3f1...c9d2"

// Attach to your outgoing HTTP request
$response = $httpClient->post($endpointUrl, [
    'headers' => [
        'Content-Type'        => 'application/json',
        'X-Webhook-Signature' => $signatureHeader,
    ],
    'body' => $payload,
]);

Receiver Side — Verifying an Incoming Webhook

use PhilipRehberger\WebhookSignature\WebhookSignature;

// Read the raw request body — do NOT decode it first
$payload   = file_get_contents('php://input');
$signature = $_SERVER['HTTP_X_WEBHOOK_SIGNATURE'] ?? '';
$secret    = 'your-shared-webhook-secret';

if (! WebhookSignature::verify($payload, $signature, $secret)) {
    http_response_code(401);
    exit('Invalid signature');
}

// Signature is valid — process the payload
$data = json_decode($payload, true);

Replay Attack Prevention

By default, signatures older than 300 seconds (5 minutes) are rejected. Adjust the tolerance for your use case:

// Accept signatures up to 60 seconds old
$valid = WebhookSignature::verify($payload, $signature, $secret, tolerance: 60);

// Disable replay protection entirely (not recommended)
$valid = WebhookSignature::verify($payload, $signature, $secret, tolerance: PHP_INT_MAX);

Exception-Based Flow with verifyOrFail()

use PhilipRehberger\WebhookSignature\WebhookSignature;
use PhilipRehberger\WebhookSignature\Exceptions\InvalidSignatureException;
use PhilipRehberger\WebhookSignature\Exceptions\SignatureExpiredException;

try {
    WebhookSignature::verifyOrFail($payload, $signature, $secret);
} catch (SignatureExpiredException $e) {
    http_response_code(401);
    exit('Signature expired');
} catch (InvalidSignatureException $e) {
    http_response_code(401);
    exit('Invalid signature');
}

Key Rotation with Multiple Secrets

When rotating webhook secrets, both the old and new secrets can be accepted during the transition period:

use PhilipRehberger\WebhookSignature\WebhookSignature;

$payload   = file_get_contents('php://input');
$signature = $_SERVER['HTTP_X_WEBHOOK_SIGNATURE'] ?? '';

// Accept signatures signed with either the old or new secret
$secrets = [
    'new-secret-after-rotation',
    'old-secret-before-rotation',
];

if (! WebhookSignature::verifyWithMultipleSecrets($payload, $signature, $secrets)) {
    http_response_code(401);
    exit('Invalid signature');
}

// Signature matched one of the secrets — process the payload
$data = json_decode($payload, true);

Algorithm-Specific Signing

Use signWith() and verifyWith() to sign and verify with SHA-384 or SHA-512 instead of the default SHA-256:

use PhilipRehberger\WebhookSignature\SignatureAlgorithm;
use PhilipRehberger\WebhookSignature\WebhookSignature;

// Sign with SHA-512
$signature = WebhookSignature::signWith($payload, $secret, SignatureAlgorithm::Sha512);

// Verify with SHA-512
$valid = WebhookSignature::verifyWith($payload, $signature, $secret, SignatureAlgorithm::Sha512);

API

Method	Description
WebhookSignature::generate(string $payload, string $secret, ?int $timestamp = null): string	Sign a payload; returns the formatted t={ts},v1={hmac} header value
WebhookSignature::verify(string $payload, string $signature, string $secret, int $tolerance = 300): bool	Verify a signature; returns false if malformed, expired, or invalid
WebhookSignature::verifyOrFail(string $payload, string $signature, string $secret, int $tolerance = 300): void	Verify a signature; throws InvalidSignatureException or SignatureExpiredException on failure
WebhookSignature::signWith(string $payload, string $secret, SignatureAlgorithm $algorithm, ?int $timestamp = null): string	Sign a payload using a specific algorithm (SHA-256, SHA-384, or SHA-512)
WebhookSignature::verifyWith(string $payload, string $signature, string $secret, SignatureAlgorithm $algorithm, int $tolerance = 300): bool	Verify a signature using a specific algorithm
WebhookSignature::verifyWithMultipleSecrets(string $payload, string $signature, array $secrets, int $tolerance = 300): bool	Verify against multiple secrets; returns true if any matches (key rotation)
WebhookSignature::parseSignatureHeader(string $signature): ?array	Parse a signature header into ['timestamp' => int, 'v1' => string]; returns null on malformed input

Development

composer install
vendor/bin/phpunit
vendor/bin/pint --test
vendor/bin/phpstan analyse

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License

MIT