As mentioned within the documentation, you should pass the date for the first day of the current month as the second parameter to the strtotime()
function:
$base = strtotime(date('Y-m',time()) . '-01 00:00:01');
echo date('m/Y',strtotime('+0 month', $base));
echo date('m/Y',strtotime('+1 month', $base));
echo date('m/Y',strtotime('+2 month', $base));
See that it works: http://ideone.com/eXis9
01/2012
02/2012
03/2012