never use '+1 month' and '-1 month' in strtotime. It's just bugged

strtotime.php

<?php
// tried this today, 31 May 2011

echo "\n";
echo date('Y-m-d', strtotime('first day of next month'));  // correct

echo "\n";
echo date('Y-m-d', strtotime('+1 month')); // wrong! output is 2011-07-01

echo "\n";
echo date('Y-m-d', strtotime('next month')); // as above, output is 2011-07-01

echo "\n";
echo date('Y-m-d', strtotime('first day of previous month'));  // correct

echo "\n";
echo date('Y-m-d', strtotime('-1 month')); // wrong! output is 2011-05-01

echo "\n";
echo date('Y-m-d', strtotime('2011-05-29 first day of previous month'));  // correct (nice!)

Does this still work in December when adding a month? @DazWilliams

It sure does James.

You can see it here:
https://extendsclass.com/php-bin/e5145f2

$i=1;

echo date("M-y", mktime(0, 0, 0, 12+$i, 1, date("Y")));

Output: Jan-22