<?php
echo "current month 28: ";
echo date('Y-m-28', mktime(0,0,0,date('m'),1,date('Y'))) . "\n";
 
echo "Next month 28: ";
echo date('Y-m-28', mktime(0,0,0,date('m')+1,1,date('Y'))) . "\n";
 
echo "Previous month 28: ";
echo date('Y-m-28', mktime(0,0,0,date('m')-1,1,date('Y'))) . "\n";