php format date string to yyyy-mm-dd code example

Example 1: php date format dd/mm/yyyy

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

Example 2: php validate date format yyyy-mm-dd

$date="2012-09-12";

if (preg_match("/^[0-9]{4}-(0[1-9]|1[0-2])-(0[1-9]|[1-2][0-9]|3[0-1])$/",$date)) {
    return true;
} else {
    return false;
}