function IDCheck() { //参数调用函数
VAR f = document.modify_form;
var Book_num = f.book_num.value;
if(book_num=="") {
window.alert("图书编号不能为空");
f.book_num.focus();
return false;
}
//加一个随机数//////////////////////////////
var number = Math.random();
number = number * 1000000000;
number = Math.ceil(number);
//////////////////////////////////////////
send_request('get_book.php?book_num='+book_num+'&ranum='+number); // 后面的 “ranum=number”是额外加的
}

<;metahttp-equiv="PRagma"content="no-cache">
<metahttp-equiv="cache-control"content="no-cache,must-revalidate">
<metahttp-equiv="expires"content="wed,26feb199708:21:57gmt">

response.expires=-1
response.expiresabsolute=now()-1
response.cachecontrol="no-cache"

header("expires:mon,26jul199705:00:00gmt");
header("cache-control:no-cache,must-revalidate");
header("pragma:no-cache");

response.addHeader("pragma", "no-cache");
response.addHeader("cache-control", "no-cache,must-revalidate");
response.addHeader("expires", "0");