เกิอ error ขึ้นแบบนี้ "Undefined constan'tool.' ต้องแก้ไขยังไง

<?php
session_start();
require "connection.php";
    $name = $_SESSION["name"];
    $tnow = date("Y-m-d H:i:s");
    foreach ($_SESSION["tool"] as $key => $value) {
    $sql = "UPDATE tools SET bname='$name', reserve=1, bdate='$tnow' WHERE id='$value'";
    $query = mysqli_query($conn, $sql);
    unset($_SESSION[tool][$key]);
    }
    header("location:borrow.php");
?>