Ok, it would be something like this:
You are using ajax to save the score and login, through a php file right?
First thing: Create a column in the user table on your mysql database for the last_loginreward_time for example.
Now, in the php file, everytime the user login, check the last_loginreward_time for the current user.
If it's not from today, update the field with the current timestamp and add the reward to the user.
If it's from today, don't update the field, and don't add the reward to the user.
Another way is creating a table just for the log of logins. Everytime a user login in the game, you create a new log with the time. Then, make a query to pull all logins from the player, of the current day, if there is more than one result, means that the user logged more than one time today, so he probably already received the reward. If there is only one (because we just added), means that this logins is the first today, so give him the reward.
I won't give you the code, because you need to know understand something that you did yourself, it's the best way to learn. But that's a logic you could use.
Edit: Sorry for answering a really old post. Was unintentional. Forgot to look at the date, my mistake.