How to send data, via POST, from android to PHP in a synchronous way?

I need to send data to the database, and I'm using PHP. I was trying through the Ion library, but because it was asynchronous, the application featured NullPointerExceptions when I had more than one data being sent. I tried to implement okHttp, but also to no avail.

EDIT: I tried to implement with Retrofit, as I was suggested, but I did not succeed. Here is the code below:

RetrofitClient

public class RetrofitClient {

    private static Retrofit retrofit = null;

    public static Retrofit getClient(String baseURL) {
        if (retrofit == null) {
            retrofit = new Retrofit.Builder().baseUrl(baseURL).addConverterFactory(GsonConverterFactory.create()).build();
        }
        return retrofit;
    }
}

PersonService

public interface PessoaService {
    @POST("/sync_pessoas")
    @FormUrlEncoded
    Call<Pessoa> savePost(@Body Pessoa post);
}

AppUtils

public class ApiUtils {

    private ApiUtils() {}

    public static PessoaService getPessoaService(String url) {
        return RetrofitClient.getClient(url).create(PessoaService.class);
    }
}

MainActivity.java

private void sendPost(Pessoa p) {
        pessoaService.savePost(p).enqueue(new retrofit2.Callback<Pessoa>() {
            @Override
            public void onResponse(retrofit2.Call<Pessoa> call, retrofit2.Response<Pessoa> response) {
                if (response.isSuccessful()) {
                    Log.i("RESPONSE", response.body().toString());
                }
            }

            @Override
            public void onFailure(retrofit2.Call<Pessoa> call, Throwable t) {
                Log.e("ERROR", "Incapaz de submeter API");
            }
        });
    }

    private void syncTablePessoasWithRetrofit() {
        pessoaService = ApiUtils.getPessoaService("http://"+Preferences.getString(MainActivity.this, "ip")+"/PesquisaPet/");
        final ProgressDialog dialog = ProgressDialog.show(MainActivity.this, "Sincronizando", "Por favor, aguarde", true, false);
        new Thread(new Runnable() {
            ArrayList<Pessoa> pessoas = (new Pessoa()).getAllNotSincronized(MainActivity.this);
            @Override
            public void run() {
                for (int i = 0; i < pessoas.size(); i++) {
                    sendPost(pessoas.get(i));
                }
                dialog.dismiss();
            }
        }).start();

    }

Person.java

public class Pessoa {

    private SQLiteDatabase db;
    private Cursor cursor;

    @SerializedName("id")
    @Expose
    private long id;
    @SerializedName("nome")
    @Expose
    private String nome;
    @SerializedName("bairro")
    @Expose
    private String bairro;
    @SerializedName("telefone")
    @Expose
    private String telefone;
    @SerializedName("cadastrado_por")
    @Expose
    private String cadastradoPor;
    @SerializedName("data_cadastro")
    @Expose
    private long dataCadastro;
}

Script php that would receive the data and send it to the bank

<?php
    include_once('database/Database.class.php');

    $db = new Database();

    $stmt = $db->query("SELECT MAX(pessoa_id) AS pessoa_id FROM Controle_id");
    $controleIDs = $stmt->fetch();

    $values = array(
        ":id" => $controleIDs->pessoa_id, 
        ":nome" => $obj->{'nome'}, 
        ":bairro" => $obj->{'bairro'}, 
        ":telefone" => $obj->{'telefone'}, 
        ":cadastrado_por" => $obj->{'cadastrado_por'}, 
        ":data_cadastro" => $obj->{'data_cadastro'}
    );
    //echo json_encode(array("success" => true, "id" => $_POST['id']));

    $stmt = $db->query("INSERT INTO Pessoas (id, nome, bairro, telefone, cadastrado_por, data_cadastro) VALUES (:id, :nome, :bairro, :telefone, :cadastrado_por, :data_cadastro)", $values);
    if ($stmt->rowCount() > 0) {
        $statement = $db->query("UPDATE Controle_id SET pessoa_id = pessoa_id + 1");
        if ($statement->rowCount() > 0) {
            echo json_encode(array("success" => true, "id" => $_POST['id'], "controle_id" => $controleIDs->pessoa_id)); 
        } else {
            echo json_encode(array("success" => true, "id" => $_POST['id']));
        }
    } else {
        echo json_encode(array("success" => false));
    }
?>

My Log shows the following errors:

Process: br.com.ufrn.marceloaugusto.pesquisapet, PID: 28476
    java.lang.StackOverflowError: stack size 1037KB
        at com.google.gson.internal.$Gson$Types.checkNotPrimitive($Gson$Types.java:434)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:551)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:553)
        at com.google.gson.internal.$Gson$Types.canonicalize($Gson$Types.java:111)
        at com.google.gson.internal.$Gson$Types$WildcardTypeImpl.<init>($Gson$Types.java:546)
        at 
07-31 09:36:44.017 28476-28476/br.com.ufrn.marceloaugusto.pesquisapet E/WindowManager: android.view.WindowLeaked: Activity br.com.ufrn.marceloaugusto.pesquisapet.MainActivity has leaked window com.android.internal.policy.PhoneWindow$DecorView{afd68b9 V.E...... R......D 0,0-684,321} that was originally added here
        at android.view.ViewRootImpl.<init>(ViewRootImpl.java:368)
        at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:299)
        at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:86)
        at android.app.Dialog.show(Dialog.java:319)
        at android.app.ProgressDialog.show(ProgressDialog.java:116)
        at android.app.ProgressDialog.show(ProgressDialog.java:104)
        at br.com.ufrn.marceloaugusto.pesquisapet.MainActivity.syncTablePessoasWithRetrofit(MainActivity.java:131)
        at br.com.ufrn.marceloaugusto.pesquisapet.MainActivity.onOptionsItemSelected(MainActivity.java:95)
        at android.app.Activity.onMenuItemSelected(Activity.java:2986)
        at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:368)
        at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:195)
        at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:108)
        at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:108)
        at android.support.v7.app.ToolbarActionBar$2.onMenuItemClick(ToolbarActionBar.java:65)
        at android.support.v7.widget.Toolbar$1.onMenuItemClick(Toolbar.java:202)
        at android.support.v7.widget.ActionMenuView$MenuBuilderCallback.onMenuItemSelected(ActionMenuView.java:780)
        at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
        at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
        at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:963)
        at android.support.v7.widget.ActionMenuView.invokeItem(ActionMenuView.java:624)
        at android.support.v7.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:150)
        at android.view.View.performClick(View.java:5217)
        at android.view.View$PerformClick.run(View.java:21198)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5451)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

EDIT 2: I tried to implement also with Volley, following a tutorial, but I could not make the request synchronous.

MainActivity.java

private void syncTablePessoasWithVolley() {
        final ProgressDialog progressDialog = new ProgressDialog(this);
        progressDialog.setMessage("Suncronizando..");
        progressDialog.show();
        final ArrayList<Pessoa> pessoas = (new Pessoa()).getAllNotSincronized(this);
        for (int i=0; i < pessoas.size(); i++) {
            final int finalI = i;
            StringRequest stringRequest = new StringRequest(Request.Method.POST, "http://" + Preferences.getString(MainActivity.this, "ip") + "/PesquisaPet/sync_pessoas.php", new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {
                    progressDialog.dismiss();
                    try {
                        Log.i("STRING_RESPONSE", "["+response+"]");
                        JSONObject obj = new JSONObject(response);
                        if (!obj.getBoolean("success")) {
                            Log.d("RESPONSE_ID", obj.getString("id"));
                        } else {
                            Log.d("JSON_RESPONSE", "Erro");
                        }
                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }
            }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    progressDialog.dismiss();
                    Log.d("JSON_RESPONSE", "ErroResponse");
                }
            }) {
                @Override
                protected Map<String, String> getParams() throws AuthFailureError {
                    Map<String, String> params = new HashMap<>();
                    params.put("id", String.valueOf(pessoas.get(finalI).getId()));
                    params.put("nome", pessoas.get(finalI).getNome());
                    params.put("bairro", pessoas.get(finalI).getBairro());
                    params.put("telefone", pessoas.get(finalI).getTelefone());
                    params.put("cadastrado_por", pessoas.get(finalI).getCadastradoPor());
                    params.put("data_cadastro", String.valueOf(pessoas.get(finalI).getDataCadastro()));
                    return params;
                }
            };

            VolleySingleton.getInstance(this).addToRequestQueue(stringRequest);
        }
    }

VolleySingleton.java

import android.content.Context;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.toolbox.Volley;

public class VolleySingleton {

    private static VolleySingleton mInstance;
    private RequestQueue mRequestQueue;
    private static Context mCtx;

    private VolleySingleton(Context context) {
        mCtx = context;
        mRequestQueue = getRequestQueue();
    }

    public static synchronized VolleySingleton getInstance(Context context) {
        if (mInstance == null) {
            mInstance = new VolleySingleton(context);
        }
        return mInstance;
    }

    public RequestQueue getRequestQueue() {
        if (mRequestQueue == null) {
            mRequestQueue = Volley.newRequestQueue(mCtx.getApplicationContext());
        }
        return mRequestQueue;
    }

    public <T> void addToRequestQueue(Request<T> req) {
        getRequestQueue().add(req);
    }
}