how to write date dd/mm/yyyy to yyyy-mm-dd in php code example

Example 1: php date format dd/mm/yyyy

date("d/m/Y", strtotime($str));

Example 2: convert dd/mm/yyyy to yyyy-mm-dd in mysql php

$var = '20/04/2012';
$date = str_replace('/', '-', $var);
echo date('Y-m-d', strtotime($date));