PHP - Insert date into mysql

I am trying to insert date into mysql but everytime it fails and comes out as 0000-00-00 in phpmyadmin

My date format is like 2012-08-06 (yyyy-mm-dd) and the date field type in database is date.

$date = "2012-08-06";
mysql_query("INSERT INTO data_table (title, date_of_event) 
             VALUES('". $_POST['post_title'] ."',
                    '". $date ."')") or die(mysql_error()); 

tried changing - to / or removing them, it doesn't work.