use explode with "/" as the separator. You will get an array with days as the first element and so on.
$first_date = explode("/", $_POST['FirstDate'])
MySQL dates follow the format (this is the principal format...I guess) YYYY-MM-DD. So you can insert that into a date field as
$mysql_date = $first_date[2] . "-" . $first_date[1] . "-" . $first_date[0];
To retrieve it, use the MYSQL function UNIX_TIMESTAMP(date_column) which gives you Unix time (number of seconds since Jan 1, 1970.) Use can then use the date function to format the value back to dd/mm/yyyy format.