try this
$dates = explode("/","03/15/2014");
$newdate = $dates[2]."-".$dates[0]."-".$dates[1];
This works in all version of php
Note: the input date should be of format mm/dd/yyyy
If your php is less than 5 you can use this. If php > 5
then it advised to use Datetime class as advised by GordonM
in order to find the date is valid or not .