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[&#39;i&#39;])){
	$_SESSION[&#39;i&#39;] = i();//获取库存
}

if($_SESSION[&#39;i&#39;] > 0){
	$buy = true;
	$_SESSION[&#39;i&#39;]--;
}else{
	$buy = false;
}

session_write_close();

if(isset($_COOKIE[&#39;PHPSESSID&#39;])){//还原原来的session
	session_id($_COOKIE[&#39;PHPSESSID&#39;]);
	session_start();
}