Constructors in PHP-Codeigniter

I solved this issue in the following way:

1. I made a new core class called "MY_Controller" which extends the CI controller class.
2. I wrote a doAuth() method which authenticates user or reroutes him to a login controller.
3. Now I call this method from all methods witch must be safe

It seems a lot less practical than other aproaches, but it saves you some work if you decide you need another controller to feature authentication.

To answer your question in a line yes you can do. Create library and add a function as i have done below and load the library within the constructor and make a call to that function. I assume you set a signal when a user login to your site like user_login within the session. Hope this answers your question.

// Place this function in the user library
  check_user_login(){
  $obj = get_instance();
   if($obj->session->userdata('user_logged') != true):
  // redirect to login page
   endif;
 }    
<?php
class New extends CI_Controller
{
 function __construct()

{

//Constructor codes...
//load the library,
$this->load->library("library_name");
// call the function name
check_user_login();

}

 function Create_page()  //The user need to be Logged in to perform this
{
 //Checking whether the user logged in or not if yes allowed else denied. 
}
 function Edit_page()      //The user need to be Logged in to perform this
{
 //Checking whether the user logged in or not if yes allowed else denied.
}
function Delete_page()  //The user need to be Logged in to perform this
{
 //Checking whether the user logged in or not if yes allowed else denied.
}
function about_us()   //This is a public action no need to Log in 
{
   // this is a pulic action ,no need to check the login status
}
}
?> 

if you give your constructor the password and username and than check if its loggend in you van save it in a var ( true/false )

class newclass{

 public function __construct($username,$password){

  $query = mysql_query("SELECT * FROM accounts WHERE user = '".$username."' AND pass = '".$password."'");
  if(mysql_num_rows($query) > 0){ // if username and password match in database your loggend in
   $this->loginCheck = true;
  }
  else{ // if not match not logged in
   $this->loginCheck = false;
  }
 }//end constructor

 public function another(){
  if($this->loginCheck){
   return "logged in";
  }
  else{
   return "nog logged in";
  }

 }// end another function

}//end class

$class = new newclass($user,$pass);
echo $class->another();

Basically, you should separate those private methods you want to be used by a logined user and the public methods which can be accessed by everyone. And if you really want to do, you can do something like the following methods. Moreover, you can modify the auth_method() to satisfy your needs with more authentications.

class Page extends CI_Controller {

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

public function create_page()
{
    // ...
}

public function edit_page()
{
    // ...
}

public function delete_page()
{
    // ...
}

public function about_us()
{
    echo 'About Us Page';
}

private function auth_method()
{
    $protected_methods  = array('create_page', 'edit_page', 'delete_page');
    $segs   = $this->uri->segment_array();
    $method = isset($segs[1]) ? trim($segs[1]) : 'index';
    if(in_array($method, $protected_methods))
        exit("Access Denied.");
}

}
//END CLASS