This example shows you how to add one day, one month or one year to todays date using PHP Date Functions.
for more information refer PHP.net

<?php

//PHP Example code to add one day,one month or one year to todays date

$todayDate = date("Y-m-d");// current date
echo "Today: ".$todayDate."<br>";

//Add one day to today
$dateOneDayAdded = strtotime(date("Y-m-d", strtotime($todayDate)) . "+1 day");

echo "After adding one Day: ".date('l dS \o\f F Y', $dateOneDayAdded)."<br>";

//Add one month to today
$dateOneMonthAdded = strtotime(date("Y-m-d", strtotime($todayDate)) . "+1 month");

echo "After adding one month: ".date('l dS \o\f F Y', $dateOneMonthAdded)."<br>";

//Add one Year to today
$dateOneYearAdded = strtotime(date("Y-m-d", strtotime($todayDate)) . "+1 year");

echo "After adding one Year: ".date('l dS \o\f F Y', $dateOneYearAdded)."<br>";
?>