mysqli函数不插入值

I'm trying to connect to a database by mysqli in an object oriented way. I had a few errors, and solved them, but now I just can solve this one. I've got my code here, and all the names (database name, user, password, host, and table names) are correct (actually, copied and pasted), but the query still returns 0.

<?php


class DbConnection
{
    public $link;

    public function __construct()
    {
        $this->link = new mysqli("localhost","root","","todo");
        if (mysqli_connect_errno()) {
            printf("Connect failed: %s
", mysqli_connect_error());
            exit();
        }
    }

    function RegisterUsers($username, $password, $ip, $name, $email)
    {
        $stmt = $this->link->prepare("INSERT INTO users (Username, `Password`, ip, Name, Email) VALUES (?,?,?,?)");
        $stmt->bind_param("sssss", $username, $password, $ip, $name, $email);
        $stmt->execute();

        $stmt->store_result();
        $count = $stmt->num_rows;
        return $count;
    }

}

$dbConn = new DbConnection();
echo $dbConn->RegisterUsers("a","a","a","a", "a");


?>

Edit: With this code, i get an

Call to a member function bind_param() on boolean

error.

Password and name are keywords in mysql. You have to put it in backticks to escape it, if you will use it as column name

    $stmt = $this->link->prepare("INSERT INTO users (Username, `Password`, ip, `Name`) VALUES (?,?,?,?)");