php - PDO connection object not passing to a class -

ok, 2 separate php files, 1 main script, other class. i'm trying connect sql database pdo. create pdo connection in main script, pass class in other script, queries made. however, getting following error :

fatal error: call member function prepare() on non-object in /applications/xampp/xamppfiles/htdocs/test/class.php on line 16 

my understanding php pass references objects across scripts, error says object not exist.

below main php script, creates pdo object , passes class played with.

<?php session_start(); unset($_session['reference']);  require('class.php');  //connect database, know username , password not included  $dsn = 'mysql:dbname=test;host=localhost';  try {     $dbh = new pdo($dsn);     $dbh->setattribute(pdo::attr_errmode, pdo::errmode_exception); } catch (pdoexception $e) {   die ( 'connection failed: ' . $e->getmessage() ); }  $objreference = new reference($dbh); $reference = $objreference->getreference();   $_session['reference'] = $reference; session_write_close();  ?> 

below class.php file. receives object , data.

<?php  class reference  {      private $reference;     private $dbh;      public function __construct($dbh) {         $this->dbh=$dbh;         $this->setreference();     }      private function setreference() {             $refid=$dbh->prepare(" select max(id) createx ");              $refid->execute();             $id = $refid->fetchcolumn();             $id=$id+1;             $this->reference = $id;     }      public function getreference() {          return $this->reference;     }  }  ?> 

is there way pass pdo objects classes not aware of? all.

since $dbh instance variable, in setreference function should using pseudo $this reference it:

$refid=$this->dbh->prepare(" select max(id) createx ");  

i.e. use $this->dbh.