<?php
// 1 week
$timestamp = strtotime('last week');
echo 'semana pasada: ' . gmdate("Y-m-d\TH:i:s\Z", $timestamp) . '<br>';

// Today
echo 'Hoy: ' . date('Y-m-d H:i:s') . '<br>';

// Yesterday
echo 'Ayer: '.date('Y-m-d H:i:s',strtotime("-1 days"));

// Tomorrow
echo 'Mañana: '.date('Y-m-d H:i:s',strtotime("+1 days"));
?>