php的session默认是以文件方式保存在服务器端，并且在客户端使用cookie保存变量，这就会出现一个问题，当一个用户由于某种安全原因关闭了浏览器的cookie，程序中的session相关操作将无法执行。因此，如果能以数据库保存session数据，将不受客户端设置的限制，并且在性能和扩展性等方面有一个飞跃。程序中使用关键函数是session_set_save_handler，同时要将php.ini里的session.save_handler = files 改为 user。这里我们讨论的环境是linux(freesd)+apache+mysql+php。

数据表结构：[sessions]
CREATE TABLE sessions (  
sesskey char(32) not null,  
expiry int(11) unsigned not null,  
value text not null,  
PRIMARY KEY (sesskey)  
);

程序代码：[session_inc.php]
<?php
$SESS_DBHOST = "yourhost"; /* database server hostname */
$SESS_DBNAME = "yourdb"; /* database name */
$SESS_DBUSER = "youruser"; /* database user */
$SESS_DBPASS = "yourpassword"; /* database password */

$SESS_DBH = "";
$SESS_LIFE = get_cfg_var("session.gc_maxlifetime");

function sess_open($save_path, $session_name) {
    global $SESS_DBHOST, $SESS_DBNAME, $SESS_DBUSER, $SESS_DBPASS, $SESS_DBH;

    if (! $SESS_DBH = mysql_pconnect($SESS_DBHOST, $SESS_DBUSER, $SESS_DBPASS)) {
        echo "<li>Can't connect to $SESS_DBHOST as $SESS_DBUSER";
        echo "<li>MySQL Error: " . mysql_error();
        die;
    }

    if (! mysql_select_db($SESS_DBNAME, $SESS_DBH)) {
        echo "<li>Unable to select database $SESS_DBNAME";
        die;
    }

    return true;
}

function sess_close() {
    return true;
}

function sess_read($key) {
    global $SESS_DBH, $SESS_LIFE;

    $qry = "SELECT value FROM session_tbl WHERE sesskey = '$key' AND expiry > " . time();
    $qid = mysql_query($qry, $SESS_DBH);

    if (list($value) = mysql_fetch_row($qid)) {
        return $value;
    }

    return false;
}

function sess_write($key, $val) {
    global $SESS_DBH, $SESS_LIFE;

    $expiry = time() + $SESS_LIFE; //过期时间
    $value = addslashes($val);

    $qry = "INSERT INTO session_tbl VALUES ('$key', $expiry, '$value')";
    $qid = mysql_query($qry, $SESS_DBH);

    if (! $qid) {
        $qry = "UPDATE session_tbl SET expiry = $expiry, value = '$value' WHERE sesskey = '$key' AND expiry > " . time();
        $qid = mysql_query($qry, $SESS_DBH);
    }

    return $qid;
}

function sess_destroy($key) {
    global $SESS_DBH;

    $qry = "DELETE FROM session_tbl WHERE sesskey = '$key'";
    $qid = mysql_query($qry, $SESS_DBH);

    return $qid;
}

function sess_gc($maxlifetime) {
    global $SESS_DBH;

    $qry = "DELETE FROM session_tbl WHERE expiry < " . time();
    $qid = mysql_query($qry, $SESS_DBH);

    return mysql_affected_rows($SESS_DBH);
}

session_set_save_handler(
"sess_open",
"sess_close",
"sess_read",
"sess_write",
"sess_destroy",
"sess_gc");

session_start();
?>

完成以上步骤后，在程序中使用require("session_inc.php")来代替session_start()即可，其他的session函数还是象以前一样的方法调用。


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