php convert date from dd/mm/yyyy to yyyy-mm-dd

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

$date = DateTime::createFromFormat('d/m/Y', "24/04/2012");
echo $date->format('Y-m-d');

lastconnection = datetime.strptime("21/12/2008", "%d/%m/%Y").strftime('%Y-%m-%d')

SELECT CONVERT(varchar(10), CONVERT(date, '13/12/2016', 103), 120)

<?php
// both lines output 813470400
echo strtotime("19951012"), "\n",
     strtotime("12 October 1995");

// prints 1995 Oct 12
echo date("Y-m-d", strtotime("19951012"));
?>