Date type: DATE

Hi everybody.

I've got a form where I ask for the date.I store the date in 3 variables: for
the day i have $day (dd), for the month i have $month (mm) and for the year
$year (yyyy).
At the same time i have a table which is as it follows:
Table "s_objetivos_caso"
Column | Type | Modifiers
-----------------+---------+-----------
dni | integer | not null
fecha | date | not null
cod_s_objetivos | integer | not null

How do i have to insert the column date?.

$date="$year-$month-$day";
insert into s_objetivos_caso values ($dni,$date,$cod_s_objetivos,$conn);

Can anyboy help me for how a i must format the date for introduce into the
table???

Thank you. And sorry with my English.