PDO UPDATE 似乎没有存储报价?

I'm just starting out with PDO methods and now I'm stuck at a little question. If I create a form to insert first name and last name into a database I can insert all types of special characters with the code below:

try {
    $db = new PDO('mysql:dbhost=' . $dbhost . ';dbname=' . $dbname, $dbuser, $dbpass);
    $db -> exec("SET CHARACTER SET utf8");
} catch(PDOException $e) {
    echo $e->getMessage();
}
$query = $db->prepare("INSERT INTO users(fname, lname) VALUES(:fname, :lname)");
$insert_array = array(
    ":fname" => $fname,
    ":lname" => $lname
);
$query->execute($insert_array);
$db = NULL;

我可以插入 ":;,-!"#¤%&&(%)?{][]}£$€{{{$@@_--"任何问题，甚至插入 SQL 注入.但是当我尝试使用类似代码更新数据库时，它确实接受所有类型的特殊字符，引号除外.为什么会这样?我用来更新的代码是:

try {
    $db = new PDO('mysql:dbhost=' . $dbhost . ';dbname=' . $dbname, $dbuser, $dbpass);
    $db -> exec("SET CHARACTER SET utf8");
} catch(PDOException $e) {
    echo $e->getMessage();
}
$query = $db->prepare("UPDATE users SET fname=:fname, lname=:lname WHERE userid=:userid");
$update_array = array(
    ":fname" => $fname,
    ":lname" => $lname,
    ":userid" => $_GET['userid']
);
$query->execute($update_array);
$db = NULL;

<form action="" method="post">
    First name<br><input type="text" name="fname" value="'.htmlspecialchars($user['fname']).'">
    Last name: <br><input type="text" name="lname" value="'.htmlspecialchars($user['lname']).'">
    <input type="submit">
</form>

<form action="" method="post">
    First name<br><input type="text" name="fname" value="<?php echo htmlentities( $user['fname'], ENT_COMPAT, 'UTF-8' ); ?>">
    Last name: <br><input type="text" name="lname" value="<?php echo htmlentities( $user['lname'], ENT_COMPAT ); ?>">
    <input type="submit">
</form>