How to get previous month and year relative to today, using strtotime and date?

Have a look at the DateTime class. It should do the calculations correctly and the date formats are compatible with strttotime. Something like:

$datestring='2011-03-30 first day of last month';
$dt=date_create($datestring);
echo $dt->format('Y-m'); //2011-02

if the day itself doesn't matter do this:

echo date('Y-m-d', strtotime(date('Y-m')." -1 month"));