<?php
$string = 12/09/2012
$ano = explode($string);
$ano = $ano[2].'/'.$ano[1].'/'.$ano[0];
print_r($ano);
?>

<?php
$string = '12/09/2012';
$ano = explode('/', $string);
$ano = $ano[2] $ano[1] $ano[0];
print_r($ano);
?>

<?php
$string = '12/09/2012';
$ano = explode('/', $string);

$ano = $ano[0].'/'.$ano[1].'/'.$ano[2];
print_r($ano);
?>

<?php
$string = '12/09/2012';
$ano = explode('/', $string);

$ano = $ano[2].'/'.$ano[1].'/'.$ano[0];
print_r($ano);
?>

<?php
$string = '12/09/2012';
$ano = explode('/', $string);

$ano = $ano[2].'/'.$ano[0].'/'.$ano[1];
print_r($ano);
?>