Php insert Server

<?php
 
 if($_SERVER['REQUEST_METHOD']=='POST'){
 $username = $_POST['username'];
 $email = $_POST['email'];
 $password = $_POST['password'];
 
 define('HOST','localhost');
 define('USER','root');
 define('PASS','root');
 define('DB','volleyDb');
 
 $con = mysqli_connect(HOST,USER,PASS,DB) or die('Unable to Connect');
 // Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 
echo "Connected successfully,";
 
 $sql = "INSERT INTO register (username,password,email) VALUES ('$username','$email','$password')";

 if(mysqli_query($con,$sql)){
 echo "Successfully Inserted";
 }else{
 echo "Could not Insert";
 }
 }else{
echo 'error';
}
?>


...........................volley android...........................


   private void registerUser() {
        final String username = editTextUsername.getText().toString().trim();
        final String password = editTextPassword.getText().toString().trim();
        final String email = editTextEmail.getText().toString().trim();

        StringRequest stringRequest = new StringRequest(Request.Method.POST, REGISTER_URL,
                new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        Toast.makeText(MainActivity.this,response,Toast.LENGTH_LONG).show();
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        Toast.makeText(MainActivity.this,error.toString(),Toast.LENGTH_LONG).show();
                    }
                }){
            @Override
            protected Map<String,String> getParams(){
                Map<String,String> params = new HashMap<>();
                params.put(KEY_USERNAME,username);
                params.put(KEY_PASSWORD,password);
                params.put(KEY_EMAIL, email);
                return params;
            }

        };

        RequestQueue requestQueue = Volley.newRequestQueue(this);
        requestQueue.add(stringRequest);
    }