Use JWK from JSON with the lcobucci/jwt library through phpseclib. Credit to @whatTool for helping with the implementation

jwk_with_phpseclib_test.php

<?php

//REQUIREMENTS!
//"phpseclib/phpseclib": "^3.0"
//"lcobucci/jwt": "^5.0"
require_once __DIR__ . '/vendor/autoload.php';

$json = '{
  "keys": [
    {
        "kid":"my key id",
        "nbf":1493763266,
        "use":"sig",
        "kty":"RSA",
        "e":"something something JWK stuff",
        "n":"something something JWK stuff"
    }
  ]
}';

function getKeyFromJwkJson(string $kid, string $jwkKeySet): ?\Lcobucci\JWT\Signer\Key
{
    $data = json_decode($jwkKeySet, true, 512, JSON_THROW_ON_ERROR);
    if (
        !is_array($data) ||
        !array_key_exists('keys', $data) ||
        !is_array($data['keys']) ||
        empty($data['keys'])
    ) {
        return null;
    }
    foreach ($data['keys'] as $key) {
        //TODO: handle nbf on keys
        if (($key['kid'] ?? '') == $kid) {
            $publicKey = \phpseclib3\Crypt\PublicKeyLoader::load(
                \phpseclib3\Crypt\RSA\Formats\Keys\JWK::load(
                    json_encode($key, JSON_THROW_ON_ERROR)
                )
            );
            return \Lcobucci\JWT\Signer\Key\InMemory::plainText(
                $publicKey->toString('pkcs8')
            );
        }
    }
    return null;
}

$key = getKeyFromJwkJson('my key id', $json);

if ($key === null) {
    //no key found
    exit(1);
}

$constraints = [
    new \Lcobucci\JWT\Validation\Constraint\SignedWith(
        new \Lcobucci\JWT\Signer\Rsa\Sha256(),
        $key
    ),
];