用准备好的语句添加的数据可以'；不能用WHERE搜索

我使用精简语句添加到SQLite3数据库中的数据无法使用WHERE进行搜索：

SELECT Active FROM Users WHERE Username="john"

我有一个PHP演示，它用一个准备好的直接语句添加数据，然后尝试搜索它们。

我的问题有两个：

1. 为什么会发生这种情况
2. 如何搜索通过准备好的语句添加的数据

这是PHP脚本。

<?php
error_reporting(E_ALL);
date_default_timezone_set('Europe/Helsinki');
ini_set('default_charset', 'UTF-8');
mb_internal_encoding("UTF-8");
header('Content-Type: text/html; charset=UTF-8');
$timezone = date('Z');
$db = '';
// ---
//
// adds a user in the db with a prepared statement
//
function add_user1($name, $pass)
{
    global $timezone;
    global $db;
    $time = time();
    try
    {
        $statement = "INSERT INTO Users (Username, Password, Time, Timezone, Active) VALUES     (:Username,:Password,:Time,:Timezone,:Active);";    
        $query = $db->prepare($statement);
        $query->bindValue(':Username', $name, SQLITE3_TEXT);
        $query->bindValue(':Password', $pass, SQLITE3_TEXT);
        $query->bindValue(':Time', $time, SQLITE3_INTEGER);
        $query->bindValue(':Timezone', $timezone, SQLITE3_INTEGER);
        $query->bindValue(':Active', '1', SQLITE3_INTEGER);
        $ok = $query->execute();
    }
    catch(PDOException $exception)
    {
        echo $exception->getMessage();
    }
}
//
// adds a user in the db with a direct execution
//
function add_user2($name, $pass)
{
    global $timezone;
    global $db;
    $time = time();
    try
    {
        $db->exec('INSERT INTO Users (Username, Password, Time, Timezone, Active) VALUES ("' .     $name . '", "' . $pass . '", ' . $time . ', ' . $timezone . ', 1);');
    }
    catch(PDOException $exception)
    {
        echo $exception->getMessage();
    }
}
//
// seeks a password for a given username
//
function seek($user)
{
    global $timezone;
    global $db;
    try
    {
        // previous tests showed that this doesn't work on all cases
        $result = $db->query('SELECT Password FROM Users WHERE Username="'. $user . '"');
        foreach ($result as $row)
        {
            $password = $row['Password'];
            echo "search through SQLite: password for $user is $password'n";
        }
        $result = $db->query("SELECT * FROM Users");
        foreach($result as $row)
        {
            $username = $row['Username'];
            $password = $row['Password'];
            if ($username == $user)
            {
                echo " search through array: password for $username is $password";
                break;
            }
        }
    }
    catch(PDOException $exception)
    {
        echo $exception->getMessage();
    }
}
// ---
echo "<pre>'n";
try
{
    $db = new PDO('sqlite::memory:');
    $db->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
                                                                            $db->exec("CREATE     TABLE     IF     NOT         EXISTS         Users     (Id             INTEGER         PRIMARY     KEY,         Username     TEXT     UNIQUE     NOT NULL, Password TEXT NOT NULL, Time INTEGER UNIQUE NOT NULL, Timezone INTEGER NOT NULL, Active BOOLEAN NOT NULL);");
}
catch(PDOException $exception)
{
    echo $exception->getMessage();
}
add_user1("Bob", "cat");
sleep(1);
add_user1("Mark", "dog");
sleep(1);
add_user2("John", "mouse");
sleep(1);
add_user2("Alice", "rodent");
try
{
    $result = $db->query('SELECT * FROM Users');
    foreach ($result as $row)
    {
        echo "      Id: " . $row['Id'] . "'n";
        echo "Username: " . $row['Username'] . "'n";
        echo "Password: " . $row['Password'] . "'n";
        echo "    Time: " . $row['Time'] . "'n";
        echo "Timezone: " . $row['Timezone'] . "'n";
        echo "  Active: " . $row['Active'] . "'n";
        echo "'n";
    }
}
catch(PDOException $exception)
{
    echo $exception->getMessage();
}
seek("Alice");
echo "'n'n";
seek("Mark");

$db = NULL;
?>