- class_exists: documentation ( source)
<?php
if (class_exists('DateTime')) {
$date = new DateTime('2013-10-31');
$date->modify('-1 month');
print "2013-10-31, -1 month: {$date->format('m')}";
$date = new DateTime('2013-10-31');
$date->modify('first day of 1 month ago');
print "2013-10-31, first day of 1 month ago: {$date->format('m')}";
} else {
print 'Not supported.';
}