Android: login and getting PHPSESSID from the response from the server?

Question

I'm such a newbie on Android and knowledge between client-server.
I'm making an Android app in which i need to login (like any social network). I already made the registration activity saving data with MySQL on my DB. The problem comes with the login. I can verify if username and password are correct but in order to don't have to login again untill server session is closed i need so save the PHPSESSID from the server and check it everytime I want to see a "protected" activity like the Home of my social network or the profile.
If the session cookie is not anymore on the server i need to login again so a new session cookie will be created.
My questions are:
How can I get the PHPSESSID from the login script?
Is it correct to save it in the sharedPreferences in order to send it to check it from the server?
Here's my code unitll now:

`public String sendPostRequest(String requestURL, HashMap postDataParams) {

    URL url;
    String response = "";
    try {
        url = new URL(requestURL);

        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setReadTimeout(15000);
        conn.setConnectTimeout(15000);
        conn.setUseCaches(true);
        conn.setRequestMethod("POST");
        conn.setDoInput(true);
        conn.setDoOutput(true);


        OutputStream os = conn.getOutputStream();
        BufferedWriter writer = new BufferedWriter(
                new OutputStreamWriter(os, "UTF-8"));
        writer.write(getPostDataString(postDataParams));

        writer.flush();
        writer.close();
        os.close();
        int responseCode = conn.getResponseCode();

        if (responseCode == HttpsURLConnection.HTTP_OK) {
            BufferedReader br = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            response = br.readLine();
        } else {
            response = "Error";
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

    return response;
}

private String getPostDataString(HashMap<String, String> params) throws UnsupportedEncodingException {
    StringBuilder result = new StringBuilder();
    boolean first = true;
    for (Map.Entry<String, String> entry : params.entrySet()) {
        if (first)
            first = false;
        else
            result.append("&");

        result.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
        result.append("=");
        result.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
    }

    return result.toString();
}`