php用session实现多并发抢购不多卖

php代码：

<?php

session_id(md5($id));//用id做为sessionid
session_start();

if(!isset($_SESSION['i'])){
	$_SESSION['i'] = i();//获取库存
}

if($_SESSION['i'] > 0){
	$buy = true;
	$_SESSION['i']--;
}else{
	$buy = false;
}

session_write_close();

if(isset($_COOKIE['PHPSESSID'])){//还原原来的session
	session_id($_COOKIE['PHPSESSID']);
	session_start();
}