Problem with call a function of a class twice!

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PHP

MySQL

hi every one!
I have built 2 classes:
1, DB

class DB extends MySQLi{
    function getRecordSet($sql){
       ...
       //Return record set
    }
}

2, Department

class Department{
    var $db;
    function getDepartment(){
        return $this->db->getRecordSet('...');
    }
}

Problem:

$dept=new Department();
$db=new DB();
$dept->db=$db;
//Problem
$department=$dept->getDepartment();
$department2=$dept->getDepartment();

My problem is that variable $department has record set, but $department2 is bool(false). I use this because I need to use departments twice.

Note: I already test another way that store departments getting the first time in another attribute, but cannot fetch.
Please help me!
Thanks!