<?php

function convData($i){
   $i = str_replace('/','-',$i);
   return str_replace('-','/',strlen(end(explode('-',$i))) == 4 ?
   date("Y-m-d", strtotime($i)) : date("d-m-Y", strtotime($i)));
}


echo convData('21/04/2017');
echo '-----';
echo convData('2017/04/21');