พี่ช่วยอธิบายโค้ดระบบตร้าสินค้าอันเนี้ยให้หน่อยนะค่ะ

<?php 
session_start();
require_once('../inc/config.inc.php'); 

function deleteFromCart($cartId = 0)
{
	if (!$cartId && isset($_GET['cid']) && (int)$_GET['cid'] > 0) {
		$cartId = (int)$_GET['cid'];
	}

	if ($cartId) {	
		$sql  = "DELETE FROM tbl_cart
				 WHERE ct_id = $cartId";

	
		$result = mysql_query($sql) or die("deleteFromCart() " . mysql_error());
	}
	echo "<meta http-equiv='refresh' content='1;url=cart.php' />";		
	
}

function deleteAbandonedCart()
{
	$yesterday = date('Y-m-d H:i:s', mktime(0,0,0, date('m'), date('d') - 1, date('Y')));
	$sql = "DELETE FROM tbl_cart
	        WHERE ct_date < '$yesterday'";
	$result = mysql_query($sql) or die("deleteFromCart() " . mysql_error());	
}
$action=$_GET[action];

echo "$action<hr>";
switch ($action) {
	case "add" :
	echo "Hello ,$action<hr>";
		
			$productId = $_GET[book_id];
			echo "$productId<hr>";
			$sql2 = "SELECT id,book_name,price,picture FROM books WHERE id = '$productId'";
			
			$result2 = mysql_query($sql2) or die("xyz " . mysql_error());
			$rs2=mysql_fetch_row($result2);
			$pd_name=$rs2[1];
			$ct_price=$rs2[2];
			$photo_prd=$rs2[3];
			echo "$productId,$pd_name,$ct_price,$photo_prd<hr>";
			$_SESSION[sid] = session_id();
			//$sid = session_id();
			$sql = "SELECT pd_id FROM tbl_cart WHERE pd_id = '$productId' AND ct_session_id = '$_SESSION[sid]'";
			$result = mysql_query($sql) or die("xyz2 " . mysql_error());
			$cartNm=mysql_num_rows($result);
			if ($cartNm == 0) {
		
		echo $_SESSION[sid];
			$sql = "INSERT INTO tbl_cart (pd_id,pd_name, ct_qty,ct_price, ct_session_id, ct_date,pd_img)
				VALUES ('$productId','$pd_name', '1','$ct_price', '$_SESSION[sid]', NOW(),'$photo_prd')";
		
			$result = mysql_query($sql) or die("xyz3 " . mysql_error());
		} else {
		
		$sql = "UPDATE tbl_cart 
		        SET ct_qty = ct_qty + 1
				WHERE ct_session_id = '$_SESSION[sid]' AND pd_id = $productId";		
				
		$result = mysql_query($sql) or die("xyz400 " . mysql_error());	
	}	

	deleteAbandonedCart();
		
	echo "<meta http-equiv='refresh' content='3;url=cart.php' />";		
		break;
	case "update" :

	$cartId     = $_POST['hidCartId'];
	$productId  = $_POST['hidProductId'];
	$itemQty    = $_POST['txtQty'];
	$numItem    = count($itemQty);
	$numDeleted = 0;
	$notice     = '';
	
	for ($i = 0; $i < $numItem; $i++) {
		$newQty = (int)$itemQty[$i];
		if ($newQty < 1) {
			
			deleteFromCart($cartId[$i]);	
			$numDeleted += 1;
		} 
			$sql = "UPDATE tbl_cart
					SET ct_qty = $newQty
					WHERE ct_id = {$cartId[$i]}";
				$result = mysql_query($sql) or die("UPDATE tbl_cart " . mysql_error());
		
	}
	echo "<meta http-equiv='refresh' content='3;url=cart.php' />";	
	
		break;	
	case "delete" :
		
		if (!$cartId && isset($_GET['cid']) && (int)$_GET['cid'] > 0) {
		$cartId = (int)$_GET['cid'];
	}

	if ($cartId) {	
		$sql  = "DELETE FROM tbl_cart
				 WHERE ct_id = $cartId";

		
		$result = mysql_query($sql) or die("deleteFromCart() " . mysql_error());
	}
	echo "<meta http-equiv='refresh' content='1;url=cart.php' />";	
		break;
}
?>