未定义的变量：寄存器用户的值

Question

$uname = $MySQLi_CON->real_escape_string(trim($_POST['user_name']));
$email = $MySQLi_CON->real_escape_string(trim($_POST['user_email']));
$upass = $MySQLi_CON->real_escape_string(trim($_POST['password']));

$new_password = password_hash($upass, PASSWORD_DEFAULT);

$check_email = $MySQLi_CON->query("SELECT email FROM users WHERE email='$email'");
$count=$check_email->num_rows;

$RandomKey = mt_rand(1,50);
**$RegKey = hash_hmac('sha512', $value, $RandomKey);** getting error Undefined variable: value line 21. This is line 21. 
$Status = 'online';
$Banned = 'no';

任何帮助都会很棒。