See That code:

DataBase.php:
<?php
class DataBase {
    private $link;

    public function __construct($host, $username, $password, $dbName){
        $this->link = mysql_connect($host, $username, $password);
        mysql_select_db($dbName, $this->link);
    }

    public function exec($query){
        $result = mysql_query($query, $this->link);
        if(is_bool($result)){
            return $result;
        }else{
            $all = array();
            while($all[] = mysql_fetch_array($result, MYSQL_ASSOC)){}
            mysql_free_result($result);
            return $all;
        }
    }

    public function close(){
        mysql_close($this->link);
    }
}
?>

Model.php:
<?php
include 'DataBase.php';

class Model {
    protected $db;

    public function __construct(){
        $this->db = new DataBase("localhost", "root", "root", "MyDB");
    }
}
?>

IndexModel.php:
<?php
include 'Model.php';

class IndexModel extends Model{

    function  __construct() {
        parent::__construct();
    }

    public function get(){
         $this->db->exec("SELECT * FROM users");
    }
}
?>

------------------------------------------------------------------------------

Parser don't see any methods of $this->db in class 'IndexModel' that extends 'Model', test it and fix this bug.
This code works fine.