Codeigniter - Interact with a Database using SQL

user.php

<?php

class User extends CI_Controller {

	public function __construct() {
		parent::__construct();
		$this->load->database();
	}

	public function insert($username, $password) {

		$sql = "INSERT INTO users (username, password) VALUES(?, ?)";
		$this->db->query($sql, array($username, $password));

		echo "Inserted";
	}

	public function select() {

		$sql = 'SELECT * FROM users';
		$result = $this->db->query($sql)->result_array();

		foreach($result as $user) {

			echo "{$user['username']} - {$user['password']}<br>";
		}
	}

	public function update($username, $password) {

		$sql = "UPDATE users SET password=? WHERE username=?";
		$this->db->query($sql, array($password, $username));

		echo "Updated";
	}

	public function delete($username) {

		$sql = "DELETE FROM users WHERE username = ?";
		$this->db->query($sql, array($username));

		echo "Deleted";
	}
}