哈希密码不匹配

我正在使用下面的函数。当我注册一个用户时，散列似乎可以正常工作。当我尝试登录时，散列不匹配。它有正确的散列，加上额外的散列。

怎么了？

function salt($pass){
$salt = 'hello';
return hash('sha512', $pass.$salt);
}
function valid_credentials($user,$pass) {
$user = mysql_real_escape_string($user);
$pass = salt($pass);
$total = mysql_query("SELECT COUNT(`user_id`) FROM `users` WHERE `user_name` = '".$user."' AND `password` = '{$pass}' ");
return (mysql_result($total, 0) == '1' ) ? true : false;
}
function add_user($user, $pass) {
$user = mysql_real_escape_string(htmlentities($user));
$pass = salt($pass);
$time = now();
mysql_query("INSERT INTO `users` ( user_name, password, date_created ) VALUES ( '{$user}', '{$pass}', '{$time}' )");
}