How to sign a JWT with ES256 in PHP

1composer require firebase/php-jwt:7.2.1

1<?php
2require __DIR__ . '/vendor/autoload.php';
3 
4use Firebase\JWT\JWT;
5 
6$privateKey = file_get_contents('key.pem'); // PEM text: PKCS#8 or "EC PRIVATE KEY"
7 
8$now = time();
9$token = JWT::encode(
10    [
11        'iss' => 'https://issuer.example.com',
12        'sub' => 'user-42',
13        'aud' => 'api://orders',
14        'iat' => $now,
15        'exp' => $now + 15 * 60,
16    ],
17    $privateKey,
18    'ES256',
19    '2026-10', // kid
20);
21echo $token, PHP_EOL;

How it works

1. composer require firebase/php-jwt:7.2.1 installs the library; vendor/autoload.php loads it.
2. The key can be the PEM text (PKCS#8 or EC PRIVATE KEY) or an OpenSSLAsymmetricKey from openssl_pkey_get_private.
3. The fourth argument of JWT::encode is the kid; a fifth, optional array adds other header fields. typ and alg are set automatically.
4. The result is the compact token string.

Gotchas

• php-jwt does not check the curve: a P-384 key with 'ES256' produces a token with a 96-byte signature that no ES256 verifier accepts. Generate P-256 keys for ES256 (prime256v1 in OpenSSL terms).
• A malformed key throws DomainException: OpenSSL unable to validate key — often a PEM with lost line breaks after passing through an environment variable.
• php-jwt keeps payload values as you give them: time() returns an integer, but a float or a numeric string would end up in the token in that form. Build iat and exp from integers.
• Verify the result once with another implementation; signing code that has never been checked by an independent verifier is the usual source of tokens only the issuer itself accepts.

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• How to compute a JWK thumbprint (RFC 7638) in PHP
• How to sign a JWT with ES256 in Node.js
• How to sign a JWT with ES256 in Ruby