<?php

$date = "1990-05-07"; // Y-m-d
$date = date("d/m/Y", strtotime($date));
echo $date; // output: 07/05/1990

echo "\n";

$date = "1990-05-07"; // Y-m-d
$date = date("Y-m-d", strtotime($date));
echo $date; // output: 1990-05-07

?>

07/05/1990
1990-05-07