formato de data do php dd / mm / AAAA
date("d/m/Y", strtotime($str));
AAAAMMDD A AAAA-mm-dd php
<?php
// both lines output 813470400
echo strtotime("19951012"), "\n",
strtotime("12 October 1995");
// prints 1995 Oct 12
echo date("Y-m-d", strtotime("19951012"));
?>
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));