UPDATE函数，用户ID未知

以下脚本更新数据库。问题。。。我不知道如何将ID更改为登录的实际用户。我目前正在使用电子邮件地址作为登录用户名。电子邮件地址作为电子邮件存储在数据库中。

<?php
    session_start();
$con=mysqli_connect("localhost", "root", "", "test");
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$firstname = mysqli_real_escape_string($con, $_POST['firstname']);
$lastname = mysqli_real_escape_string($con, $_POST['lastname']);

$sql="UPDATE users SET `firstname`='$firstname' WHERE `id???`=??#?? LIMIT 1";

if (!mysqli_query($con,$sql)) {
  die('Error: ' . mysqli_error($con));
}
echo "1 record added";
mysqli_close($con);
?>
<?php
if( !isset( $_SESSION['email']) ){
    session_start();
}
    if($_GET["logout"]==1 AND $_SESSION['id']) { session_destroy();
         $message="You have been logged out";
    }
include("connection.php");
if ($_POST['submit']=="Sign Up") {
    if (!$_POST['email']) $error.="<br />Please enter your email";
        else if (!filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) $error.="<br />Please enter a valid email address";
    if (!$_POST['password']) $error.="<br />Please enter your password";
        else {
        if (strlen($_POST['password'])<8) $error.="<br />Please enter a password with at least 8 characters";
        if (!preg_match('`[A-Z]`', $_POST['password'])) $error.="<br />Please enter at least 1 capital letter";
        }
     if ($error) $error = "There were error(s) in your signup details:".$error;
     else {

        $query = "SELECT * FROM `users` WHERE email='".mysqli_real_escape_string($link, $_POST['email'])."'";
        $result = mysqli_query($link, $query);
        $results = mysqli_num_rows($result);
        if ($results) $error = "That email address is already registered";
        else {
            $query="INSERT INTO `users` (`email`, `password`) VALUES('".mysqli_real_escape_string($link, $_POST['email'])."', '".md5(md5($_POST['email']).$_POST['password'])."')";
            mysqli_query($link, $query);
            $message = "You've been signed up!";
            $_SESSION['id']=mysqli_insert_id($link);              
            header("Location:profile.php");
        }
    }
}
 if ($_POST['submit']=="Log In") {
    $query="SELECT * FROM `users` WHERE email='".mysqli_real_escape_string($link, $_POST['loginemail'])."' AND password='".md5(md5($_POST['loginemail']).$_POST['loginpassword'])."' LIMIT 1";
    $result = mysqli_query($link, $query);
    $row = mysqli_fetch_array($result);
    if ($row) {
        $_SESSION['id']=$row['id'];
            header("Location:profile.php");
        } else {
         $loginerror = "We could not find a user with that email and password. Please try again.";
        }
    }
?>