formato de fecha php dd / mm / aaaa
date("d/m/Y", strtotime($str));
php convertir fecha de dd / mm / aaaa a aaaa-mm-dd
$date = DateTime::createFromFormat('d/m/Y', "24/04/2012");
echo $date->format('Y-m-d');
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"));
?>