Google Access Token without authorization

I'm developing module for obtaining additional data from google analytics account. To get this data Google requires access_token. This is what I've managed so far

if (isset($_GET['code'])) {
    // try to get an access token
    $code = $_GET['code'];
    $url = 'https://accounts.google.com/o/oauth2/token';
    $params = array(
        "code" => $code,
        "client_id" => "559825975819-881lg83vs8feo70v5unqa8kfoijuvfnn.apps.googleusercontent.com",
        "client_secret" => "vj4UNNItAJocX4RkNaD_3DQ4",
        "redirect_uri" => 'http://' . $_SERVER["HTTP_HOST"] . $_SERVER["PHP_SELF"],
        "access_type" => "offline",
        "grant_type" => "authorization_code"
    );

    $ch = curl_init();
    curl_setopt($ch, constant("CURLOPT_" . 'URL'), $url);
    curl_setopt($ch, constant("CURLOPT_" . 'POST'), true);
    curl_setopt($ch, constant("CURLOPT_" . 'POSTFIELDS'), $params);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $output = curl_exec($ch);
    $info = curl_getinfo($ch);
    curl_close($ch);
 $data = (json_decode($output, true));


 $access_token_var = $data['access_token'];

  echo $access_token_var;

} else {

    $url = "https://accounts.google.com/o/oauth2/auth";

    $params = array(
        "response_type" => "code",
        "client_id" => "559825975819-881lg83vs8feo70v5unqa8kfoijuvfnn.apps.googleusercontent.com",
        "redirect_uri" => 'http://' . $_SERVER["HTTP_HOST"] . $_SERVER["PHP_SELF"],
        "scope" => "https://www.googleapis.com/auth/analytics",
        "access_type" => "offline",
        "approval_prompt" => "force"
    );

    $request_to = $url . '?' . http_build_query($params);

    header("Location: " . $request_to);
}

And I'm getting access_token, it is echoing in needed variable. But I want to obtain analytics additional data in background process (for example, while client making order and clicking order button), but everytime I need new access_token, i need to authorize with my google account, and therefore, every client on web site needs to do this, despite the fact, that I set up "access_type" => "offline". What's wrong? Or is it something wrong with my API app?

from Google Access Token without authorization