This little bit of code is a great little time saver, while there are more specialised expressions that will check the different attributes of the date fall between specific numbers, this is simple and assumes the information has already been validated through checkdate or something client side.

$date = "23/06/1981";
$data = preg_replace("/(\d{2})[\/-](\d{2})[\/-](\d{4})/" , "\\3-\\2-\\1" , $data);
// $data would now be 1981-06-23

or as a function

function db_date($date) {
    $value = preg_replace("/(\d{2})[\/-](\d{2})[\/-](\d{4})/" , "\\3-\\2-\\1" , $data);
    return $value;
}

$data = db_date("23/06/1981");
// $data would be 1981-06-23

Example:

23/06/1981 returns 1981-06-23