<?php
class User {
    private $db_fields = array(
                        'id', 
                        'username', 
                        'password' => 'ismailPassword', 
                        'first_name',
                        'last_name'
      );

    function getValue($key){
       if (array_key_exists($key, $this->db_fields)){
            return $this->db_fields[$key];
       }
       return NULL;
    }

    function setValue($key, $value){
        $this->db_fields[$key] = $value;        
    }
}

$user = new User();
$user->setValue('username', 'Ismail');
echo " Username: ";
echo $user->getValue('username');
echo "\n\n Password: ";
echo $user->getValue('password');
?>

 Username: Ismail

 Password: ismailPassword