Error while converting Oracle results to JSON

I'm getting this error while retrieving data from an Oracle database table using a PHP web service: It shows the warning - unsupported json

Warning: [json] (json_encode_r) type is unsupported, encoded as null.

This is my code:

<?php 
    require_once("Classes/abc.php");
    $h=new NetworkLevel();
    $rs=$h->GetWEBSERVICE_STATS();
    //$h->Display($rs); -- commented

    echo json_encode($rs);

?> 

This is my NetworkLevel Class Code:

<?php 
require_once('connect.php'); 
class NetworkLevel 
{    
    public $fdate; 
    public $tdate; 
    public function Display($recordset)
    { 
        rs2html($recordset); 
    } 


    function GetWEBSERVICE_STATS()
    { 
        $xCon=Connect(); 
        $xCon->SetFetchMode(ADODB_FETCH_ASSOC); 
        $str="Select * from DEPT"; 
        $recordset= $xCon->Execute($str); 
        return $recordset; 
    }    

    catch (exception $e) 
    { 
        var_dump($e); 
    } 
    return $recordset; 

    }    

} 

?>