MySQL datetime versus datetime set in session. Which came first?

I'm trying to calculate whether the results i get from the database are made after the time set in session. However, it doesn't seem to work.

I set the time session like this:

$_SESSION['lastVisit'] = $user['lastVisit'];

Where $user['lastVisit'] is a datetime field in my database.

And i'm trying to select results from the database doing this:

if($unreadThreadsSql = $cms->db->query('SELECT * FROM forum_threads WHERE posted < ' . $_SESSION['lastVisit']))

I'm not sure what i'm doing wrong here, and would like your guys opinion. For reference, here's the entire code. I might just be doing something wrong somewhere.

public function __construct()
{
    $this->getUnread();
}

private function getUnread()
{
    global $cms;

    if($unreadThreadsSql = $cms->db->query('SELECT * FROM forum_threads WHERE posted < ' . $_SESSION['lastVisit']))
    {
        $forumSQL = $cms->db->query('SELECT * FROM forum_for');
        while(开发者_JAVA百科$forums = $forumSQL->fetch_assoc())
        {
            $forumId = $forums['id'];
            $forumArray[$forumId] = $forums;
        }
        while($unreadThreads = $unreadThreadSql->fetch_assoc())
        {
            $forumId = $unreadThreads['forId'];
            $cms->db->query('INSERT INTO forum_unread(threadID, catID, forumID, userID, datetime, threadtime) VALUES('.$unreadThreads['id'].', '.$forumArray[$forumId]['cat_id'].','.$forumId.','.$_SESSION['userId'].',NOW(),'.$unreadThreads['posted'].' )');
        }
    }
}

Help is appreciated guys.

What is the value of $_SESSION['lastVisit']? And shouldn't you be quoting the value, like:

"SELECT * FROM forum_threads WHERE posted > '" . $_SESSION['lastVisit'] . "'"

SELECT * FROM forum_threads WHERE posted < ' . $_SESSION['lastVisit']

Shouldn't that be > ' . $_SESSION['lastVisit']?