You can use PHP date format, but it wouldnt make much difference.
Also you can make a 'one-liner', using my last version
<?php
$startDate = '10/16/2007';
// Default version using strftime() format
$format = '%Y-%m-%d';
$startDate = strtotime($startDate);
$startDate = strftime($format, $startDate);
echo $startDate;
echo '<hr>';
$startDate = '10/16/2007';
// Using date() with PHP date format
$format = 'Y-m-d';
$startDate = strtotime($startDate);
$startDate = date($format, $startDate);
echo $startDate;
echo '<hr>';
$startDate = '10/16/2007';
// One-liner version with date()
$startDate = date('Y-m-d', strtotime($startDate));
echo $startDate;
echo '<hr>';