I have table with 4 columns: Date, John, Frank, Anthony

I want to make a form which can be filled in by these 3 people everyday and store these values in the database. When John fills in the form today, a new row should be created with the date of today, with his value in the database. When Frank fills in the form 1 hour later (so the same day) his values should also be inserted in the database but in the same row because there's already a row with today's date. But when Anthony fills in the form tomorrow, a new row should me created with the date of tomorrow.

So in short: the program checks if anyone has already filled in the form today. If yes: it just adds the value to the existing row of today in column of the person who filled it in. if not: it makes a new row with the date of today.

I already wrote this code, but the problem is that it makes a new row everytime someone fills in the form, and there should only be a row created if $person is the first one to fill in the form on that day.

$sql = "INSERT INTO table (Date, $name) VALUES (CURDATE(),'$values')"; 

First make sure that the Date field is a primary or unique key. Then you can use ON DUPLICATE KEY UPDATE

$sql = "INSERT INTO table (Date, $name) VALUES (CURDATE(),'$values') 
        ON DUPLICATE KEY UPDATE $name='$values'"; 

But you really should check for prepared statements in your language (PDO in case of PHP) to prevent SQL injections.

Try using REPLACE INTO instead of using INSERT INTO.

EDIT

I just re-read the question - the OP wants to overwrite the data if the user re-submits the form - first time I read it I thought they wanted to keep the original data

An alternative [if you only wanted to keep the original data only], would be using insert ignore : see this answer for a comparison between insert ignore and on duplicate key update: "INSERT IGNORE" vs "INSERT ... ON DUPLICATE KEY UPDATE"

Though you should probably change the table design to be 'date', 'person', 'data' as suggested in another answer

You should change your table.

Instead of a column for each person make a name column so you have:

Date | Name | Values

Make date and person the primary key:

ALTER TABLE 'table' ADD PRIMARY KEY (Date,Name)

Then insert like this:

INSERT INTO table (Date,Name,Values) VALUES (CURDATE(),'$name','$values') ON DUPLICATE KEY UPDATE Values='$values'