public class RegisterRequest extends StringRequest{
    private static final String REGISTER_URL = "https://eapps.site.com.my/Register.php";
    private Map<String, String> params;
    public RegisterRequest(String username, String password, Response.Listener<String> listener)
    {
        super(Method.POST, REGISTER_URL, listener, null);
         params = new HashMap<>();
         params.put("username", username);
         params.put("password", password);
    }
    @Override
    public Map<String, String> getParams()
    {
        return params;
    }
}

public class RegisterActivity extends AppCompatActivity {
    EditText etUsername, etPassword;
    Button bRegister;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        etUsername = (EditText)findViewById(R.id.etUsername);
        etPassword = (EditText)findViewById(R.id.etPassword);
        bRegister  = (Button)findViewById(R.id.bRegister);
        bRegister.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final String username = etUsername.getText().toString();
                final String password = etPassword.getText().toString();
                Response.Listener<String> responseListener = new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        try {
                            JSONObject jsonObject = new JSONObject(response);
                            Boolean success = jsonObject.getBoolean("success");
                            if(success){
                                Toast.makeText(RegisterActivity.this, "success", Toast.LENGTH_SHORT).show();
                            }
                            else{
                                Toast.makeText(RegisterActivity.this, "failed", Toast.LENGTH_SHORT).show();
                            }
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                    }
                };
                RegisterRequest registerRequest = new RegisterRequest(username, password, responseListener);
                RequestQueue requestQueue = Volley.newRequestQueue(RegisterActivity.this);
                requestQueue.add(registerRequest);
            }
        });
    }
}

<?php
    $connect      = mysqli_connect("localhost", "root", "root", "rozaimi");
    $username     = $_POST["username"];
    $password     = $_POST["password"];
        global $connect, $username,$password;
        $statement = mysqli_prepare($connect, "INSERT INTO users (username,password) VALUES (?, ?)");
        mysqli_stmt_bind_param($statement, "ss",$username,$password);
        mysqli_stmt_execute($statement);
        mysqli_stmt_close($statement);
    $response = array();
    $response["success"] = false;
    $response["success"] = true;
    echo json_encode($response);
?>