formato de data do php dd / mm / AAAA
date("d/m/Y", strtotime($str));
php converter a data de dd / mm / AAAA para AAAA-mm-dd
$date = DateTime::createFromFormat('d/m/Y', "24/04/2012");
echo $date->format('Y-m-d');
converter dd / mm / AAAA em AAAA-mm-dd em php mysql
$var = '20/04/2012';
$date = str_replace('/', '-', $var);
echo date('Y-m-d', strtotime($date));