php – 简单的登录检查密码

<?php
    if (isset($_POST['submitted'])) {

    $errors = array();
        require_once ('mysql_connect.php');

    session_start();
    $username = $_POST["username"]; // This is the inputted username from the form in Login.html
    $password = $_POST["password"]; // This is the inputted password from the form in Login.html


    if (empty($errors)) {
        $query="SELECT username FROM users WHERE username='$username' AND password='SHA($password)'"; 

        $result = mysql_query($query);  

        // Mysql_num_row is counting table row

        if (mysql_num_rows($result) == 1) {
                $_SESSION["username"] = $username; // Creates a cookie saving the username
                $_SESSION["loggedIn"] = true; // Creates a cookie saying the user is logged in
            // Show thank you message
            echo '<h3 style="color:green;">Thank You!</h3>
            <span style="color:green;">You have been logged in.</span>';
        } else {
            echo '<font color="red">You could not be logged in,please make sure your username and password is correct.</font>';
            foreach ($errors as $msg) {
            echo " - <font color="red">$msg</font><br />n";
            }
        }

} else {
        echo '<font color="red"><h3>Error!</h3>
        The following error(s) occured:<br /></font>';

        foreach ($errors as $msg) {
            echo " - <font color="red">$msg</font><br />n";
        }
    }
}
?>

我得到一个：

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /login.php on line 19

我的密码是否正确？