appleid_secret.php

<?php

// See https://github.com/web-token/jwt-framework/tree/v1.3.9 for about JWT

function createAppleIDSecret()
{
    $privateKey = file_get_contents('/path/to/your/key');
    $keyId = 'key_id';
    $teamId = 'team_id';
    $clientId = 'client_id';

    $jwk = JWKFactory::createFromKey($privateKey, null, [
        'kid' => $keyId,
    ]);

    $build = new JWSBuilder(null, new AlgorithmManager([
        new ES256(),
    ]));

    $arr = [
        'aud' => 'https://appleid.apple.com',
        'exp' => time() + 60,
        'iat' => time(),
        'iss' => $teamId,
        'sub' => $clientId,
    ];

    $jws = $build->withPayload(JsonConverter::encode($arr))
        ->addSignature($jwk, ['kid' => $keyId, 'alg' => 'ES256'])
        ->build();

    return (new CompactSerializer())->serialize($jws);
}