รบกวนด้วยค่าสร้างรายงานเป็น pdf แล้ว error "FPDF error: Some data has already been output, can't send PDF file"

<?php
include('fpdf.php');
include("conf/ConnectDb.php");
class PDF extends FPDF { 
//Simple table 
function BasicTable($header,$data) 
{ 
	//Header 
	foreach($header as $col) 
		$this->Cell(40,7,$col,1); 
		$this->Ln(); 
		//Data 
		$cnt1 = 0; 
		while($cnt1 < count($data)) { 
			$cnt2 = 0; 
			while($cnt2 < count($data[$cnt1])) { 
				$this->Cell(40,6,$data[$cnt1][$cnt2],1); 
				$cnt2++; 
			} 
			$this->Ln();  
		$cnt1++; 
		}
	} 
}//end class 
 
//header declare 
 $header=array('Id','Level'); 
 
//data query from database keep to array 2 dimension 
$sql = "Select IdOwner,LevelAudit From Audit"; 
$query = mssql_query($sql);
$i=0; 
while ($rss=mssql_fetch_array($query)) { 
  $data[$i][0] = $rss['IdOwner']; 
  $data[$i][1] = $rss['LevelAudit']; 
$i++; 
} 
 
//create object pdf 
$pdf = new PDF(); 
$pdf->SetFont('Arial','',14); 
$pdf->AddPage(); 
$pdf->BasicTable($header,$data); 
$pdf->Output();
?>