How to handle the uppercase & lowercase in login authentication?

Code:
&lt;?php

class User_model extends Model {

    

    function User_model()

    {

        parent::Model();

    }

    

    function check_login($username, $password)

    {

        //login_checker!!!!!!!!!!!!!

        //

        //$shal_password = sha1($password);

              

        $query_str = "SELECT * FROM users WHERE username = '$username'

                    AND password = PASSWORD('$password')";

        

        $result = $this->db->query($query_str, array($username, $password)); 

        

        //echo 'username: '.$username.'<br/>';

        //echo 'password: '.$password.'<br/>';

        //echo 'password hash: '.$shal_password.'<br/>';

        //$query_str = "SELECT * FROM users WHERE username = ? and password = ?";

        //$result = $this->db->query($query_str, array($username, $shal_password));

        //echo 'query: '.$this->db->last_query();

        //exit();  

       

           

       if ($result->num_rows() == 1)

        {     



           return $result->row(0)->teach_id;

        }

        else

        {

            return false;

        }

       

        

    }

    

    

    

}





?&gt;