- print_r: documentation ( source)
- date_create_from_format: documentation ( source)
<?php
$day = 10;
$month = 12;
$year = 88;
$format = strlen($year) == 2 ? "y-m-d" : "Y-m-d";
$dt = date_create_from_format($format, $year.'-'.$month.'-'.$day)->format("Y-m-d");
print_r($dt);