<?php 
$validDate = '01.10.2013 11:06:12';
$cur_date=date('d.m.Y');
$cur_date = explode('.',$cur_date);
$get_date = explode(' ',$validDate);
$get_date_typ = explode('.',$get_date[0]);
if (($cur_date[0]==$get_date_typ[0])&&($cur_date[1]==$get_date_typ[1])&&($cur_date[2]==$get_date_typ[2]))
{echo 'Сегодня '.$get_date[1];}
elseif(($cur_date[0]-1==$get_date_typ[0])&&($cur_date[1]==$get_date_typ[1])&&($cur_date[2]==$get_date_typ[2]))
{echo 'Вчера '.$get_date[1];}
?>