<?
$date1 = strtotime('2009-01-01');
$date2 = strtotime('2010-01-01');

while ($date1 <= $date2) {
  echo date('Y-m-d', $date1) . "\n";
  $date1 = strtotime('+1 month', $date1);
}

2009-01-01
2009-02-01
2009-03-01
2009-04-01
2009-05-01
2009-06-01
2009-07-01
2009-08-01
2009-09-01
2009-10-01
2009-11-01
2009-12-01
2010-01-01