<?php
$con = mysqli_connect("localhost", "user", "password", "dbname");

$username = $_POST["username"];
$email=$_POST["email"];
$password = $_POST["password"];

$statement = mysqli_prepare($con, "INSERT INTO `user` (`username`, `email`, `password`) VALUES (?, ?, ?)");
mysqli_stmt_bind_param($statement, "sss", $username, $email, $password);
mysqli_stmt_execute($statement);

$response = array();
$response["success"] = true;

echo json_encode($response);

public class RegisterActivity extends Activity{
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_register);

    final EditText username = (EditText) findViewById(R.id.uname);
    final EditText emailid = (EditText) findViewById(R.id.eid);
    final EditText password = (EditText) findViewById(R.id.pword);
    final EditText cpword = (EditText) findViewById(R.id.cp);
    final Button regbtn = (Button) findViewById(R.id.register);
    //final String pword = password.getText().toString();
    //final String conpword = cpword.getText().toString();

    //if(pword.equals(conpword))
        regbtn.setOnClickListener(new View.OnClickListener(){
                                      public void onClick(View v){
                                      final String uname=username.getText().toString();
                                      final String mailId=emailid.getText().toString();
                                      final String pword = password.getText().toString();


                                          Response.Listener<String> resListener=new Response.Listener<String>(){

                                          @Override
                                          public void onResponse(String response) {
                                              try {
                                                  JSONObject jsonResponse = new JSONObject(response);

                                                  Boolean success = jsonResponse.getBoolean("success");

                                                  if(success){
                                                      Intent intent = new Intent(RegisterActivity.this, MainActivity.class);
                                                      RegisterActivity.this.startActivity(intent);
                                                  }
                                                  else{
                                                      AlertDialog.Builder alertDialog = new AlertDialog.Builder(RegisterActivity.this);
                                                      alertDialog.setMessage("Registration failed")
                                                              .setNegativeButton("Retry",null)
                                                              .create()
                                                              .show();
                                                  }


                                              } catch (JSONException e) {
                                                  e.printStackTrace();
                                              }
                                          }
                                      };

                                      RegisterRequest registerRequest = new RegisterRequest(uname,mailId,pword,resListener);
                                      RequestQueue registerQueue = Volley.newRequestQueue(RegisterActivity.this);
                                      registerQueue.add(registerRequest);

                                  }

    }

    );