- var_dump: documentation ( source)
- preg_match: documentation ( source)
- sprintf: documentation ( source)
<?php
var_dump( preg_match('/^(\d{1,2})\D(\d{1,2})\D(\d{4})$/', '23/04/2015', $matches) );
var_dump( $matches );
var_dump( sprintf("%04d-%02d-%02d", $matches[3], $matches[2], $matches[1]) );