public class Register extends AppCompatActivity {

EditText ET_NAME, ET_USER_NAME, ET_USER_PASS;
String name, user_name, user_pass;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ET_NAME=(EditText)findViewById(R.id.name);
    ET_USER_NAME=(EditText)findViewById(R.id.new_user_name);
    ET_USER_PASS=(EditText)findViewById(R.id.new_user_pass);
}

public void userReg(View view){
    name=ET_NAME.getText().toString();
    user_name=ET_USER_NAME.getText().toString();
    user_pass=ET_USER_PASS.getText().toString();
    String method="Register";
    BackgroundTask backgroundTask=new BackgroundTask(this);
    backgroundTask.execute(method,name,user_name,user_pass);
}



public class BackgroundTask extends AsyncTask<String, Void, String>{
Context ctx;
BackgroundTask(Context ctx){
    this.ctx=ctx;
}


@Override
protected void onPreExecute() {
    super.onPreExecute();
}

@Override
protected String doInBackground(String... params) {
    String reg_url="http://test99.ru/register.php";
    String method=params[0];

    if(method.equals("Register")){
        String name=params[1];
        String user_name=params[2];
        String user_pass=params[3];
        try {
            URL url=new URL(reg_url);
            HttpURLConnection httpURLConnection=(HttpURLConnection)url.openConnection();
            httpURLConnection.setRequestMethod("POST");
            httpURLConnection.setDoOutput(true);
            OutputStream OS=httpURLConnection.getOutputStream();
            BufferedWriter bufferedWriter=new BufferedWriter(new OutputStreamWriter(OS,"UTF-8"));

            String data= URLEncoder.encode("user","UTF-8")+"="+URLEncoder.encode(name,"UTF-8")+"&"+
                    URLEncoder.encode("user_name","UTF-8")+"="+URLEncoder.encode(user_name,"UTF-8")+"&"+
                    URLEncoder.encode("user_pass","UTF-8")+"="+URLEncoder.encode(user_pass,"UTF-8");

            bufferedWriter.write(data);
            bufferedWriter.flush();
            bufferedWriter.close();
            OS.close();
            InputStream IS=httpURLConnection.getInputStream();
            IS.close();
            return "Registration Success...";

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    return null;
}

@Override
protected void onProgressUpdate(Void... values) {
    super.onProgressUpdate(values);
}

@Override
protected void onPostExecute(String result) {
    Toast.makeText(ctx,result,Toast.LENGTH_LONG).show();
}
}

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".Register">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".MainActivity"/>

</application>

$db_name="webappdb";
$mysql_user="root";
$mysql_pass="";
$server_name="localhost";

$con=mysqli_connect($server_name,$mysql_user,$mysql_pass,$db_name);

require "init.php";

$name=$POST["user"];
$user_name=$POST["user_name"];
$user_pass=$POST["user_pass"];

$sql_query="insert into user_info
values('$name','$user_name','$user_pass');";
mysqli_query($con,$sql_query);
?>

String reg_url="http://test99.ru/register.php";

String reg_url="http://localhost/register.php"

String reg_url="http://ip/register.php",