PHP和mysqli - 致命错误：在非对象上调用成员函数bind_param（）[重复]

I know this probably has a very easy fix,but I have been searching around for hours,looking for a solution,only to have nothing.Please don't mark this as a duplicate simply because of the title.bind_param calling to a non-object is usually a syntax error,not an easy fix error.

Code for connect.php:

<?php
    $db = new mysqli('127.0.0.1', 'root', '', 'mon');
    if($db->connect_errno) {
        echo "<p id='gotem'>Something has gone wrong.Tell J to fix it.</p>";
    }
?>

Code for user_add.php:

<?php
require_once 'connect.php';
    if(!empty($_POST)) {
            if(isset($_POST['username'], $_POST['password'])) {
                $username = $_POST['username'];
                $password = $_POST['password'];

                if(!empty($username) && !empty($password)) {
                    $insert = $db->prepare("INSERT INTO 'user' ('name', 'pass') VALUES (?, ?)");
                    $insert->bind_param('ss', $username, $password);
                    if (!$insert->execute()) {
                         echo "Execute failed: (" . $stmt->errno . ") " . $stmt->error;
                     }                
                }
            }
        }
$_SESSION['uid'] = $_POST['username'];
$_SESSION['pss'] = $_POST['password'];
if(!mysqli_query($db_con, $db_query)) {
    die('Error:'.mysqli_error($db_con));
}
header("Location: index.php");
?>

</div>

Avoid the apostrophes in INSERT statement:

                $insert = $db->prepare("INSERT INTO user (name, pass) VALUES (?, ?)");

The apostrophes should surround strings, not names of tables or columns.