รบกวนช่วยอธิบาย โค๊ดโปแกรมดังต่อไปนี้ ให้ผมเข้าใจหน่อยคับ

if(isset($_GET['id']) && isset($_SESSION['member_id'])){
	$sql = "select * from markers where id='".$_GET['id']."'";
	$query =mysql_query($sql) or die ("error=$sql");
	$objResult = mysql_fetch_array($query);	
	$check_user = $objResult['user_vote'];
	$arrayUser = explode(",",$check_user);
	if(!in_array($_SESSION['member_id'], $arrayUser)){
		$sql = "update markers set vote = vote+1, user_vote = concat(user_vote , '".$_SESSION['member_id'].",') where id = '".$_GET['id']."'";