แก้ยังไงครับ Cannot modify header information - headers already sent by

$name = mysql_result($result, 0, "name");
 	$size = mysql_result($result, 0, "size");
	$type = mysql_result($result, 0, "type");
 	$content = mysql_result($result, 0, "content");

	header("Content-Type: $type");
	header("Content-Length: $size"); 
	header("Content-Disposition: attachment; filename=$name"); 
 	
 	echo $content;
	exit();