$client = new \GuzzleHttp\Client();
$url = 'http://whatsappauth.sr/api/v1/send-otp';
$response = $client->post(
    $url,
    [
        'headers' => [
            'Content-Type' => 'application/json',
            'Accept' => 'application/json',
            'Happy-Auth-Api-Key' => 'Your Happy-Auth-Api-Key',
            'Happy-Auth-Api-Secret' => 'Your Happy-Auth-Api-Secret',
        ],
        'json' => [
            'phone_number' => '9876543210',
            'otp' => '123456',
        ],
    ]
);
$body = $response->getBody();
print_r(json_decode((string) $body));