How to send data from Android to php via POST method?

You can use Android's HttpURLConnection class for that. Following is the method to make that request with parameters.

// Create Post Request
public String makePostRequest(String requestURL, String params) {

    String text = "";
    BufferedReader reader = null;

    // Send data
    try {
        // Defined URL  where to send data
        URL url = new URL(requestURL);

        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
        conn.setRequestProperty("Accept", "*/*");

        conn.setDoOutput(true);
        OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
        wr.write(params);
        wr.flush();

        // Get the server response

        reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
        StringBuilder sb = new StringBuilder();
        String line = null;

        // Read Server Response
        while ((line = reader.readLine()) != null) {
            // Append server response in string
            sb.append(line + "\n");
        }

        text = sb.toString();
    } catch (Exception ex) {
        ex.printStackTrace();
    } finally {
        try {
            reader.close();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
    // Show response on activity
    return text;
}

The parameters passed to this method should be encoded in order to avoid any inconvenience due to special characters. Following is the method to encode parameters

public String encodeParams(HashMap<String, String> params) throws UnsupportedEncodingException {
    String encodedParams = "";
    /* Display content using Iterator*/
    Set set = params.entrySet();
    Iterator iterator = set.iterator();
    while (iterator.hasNext()) {
        if (encodedParams != null && !encodedParams.equals("")))
            encodedParams += "&";
        Map.Entry mentry = (Map.Entry) iterator.next();
        encodedParams += URLEncoder.encode(mentry.getKey().toString(), "UTF-8")
                + "=" + URLEncoder.encode(mentry.getValue().toString(), "UTF-8");
    }
    return encodedParams;
}

In order to make Http Request, call these methods as follows:

try {
            // prepare parameters for http call
            HashMap<String, String> hashMap = new HashMap<>();
            hashMap.put("serial", android.os.Build.SERIAL);
            hashMap.put("board", android.os.Build.BOARD);

            // encode parameters
            String encodedParams = encodeParams(hashMap);
            // create request url
            String requestUrl = "http://xxxxxxxxxxxx/"; // your URL
            // make a post request to server with request url create above
            String response = makePostRequest(requestUrl, encodedParams, null);
            return response;
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }

P.S. you should try running this code using background thread or AsyncTask for better performance avoiding NetworkOnMainThread exception.