That is why you should use an auto increment id for each user, two users will never have the same id, which will avoid this problem. It's more efficient this way.
If you're lazy to change your whole system, you can use the BINARY keyword in your query.
example
Код:
SELECT `username` FROM `users` WHERE BINARY `username` = 'Fernado'
or, you could just try changing the language collation to latin_gener_cs.
You don't need to change anything in the queries. Just set the table's collation setting to latin1_general_cs. It doesn't really matter which setting you choose, as long as it has the _cs at the end (which stands for Case Sensitivity).