Try
$start = $month = strtotime('2009-02-01');
$end = strtotime('2011-01-01');
while($month < $end)
{
echo date('F Y', $month), PHP_EOL;
$month = strtotime("+1 month", $month);
}
Mind the note http://php.net/manual/de/datetime.formats.relative.php
Relative month values are calculated based on the length of months that they pass through. An example would be "+2 month 2011-11-30", which would produce "2012-01-30". This is due to November being 30 days in length, and December being 31 days in length, producing a total of 61 days.
As of PHP5.3 you can use http://www.php.net/manual/en/class.dateperiod.php
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