Understanding PHP dateTime object - php

Question

I run an automated process via PHP cron job every day which generates a report for a number of months. The date generation section of the code is below with a bit of hard coding for simplicity.
On the last day of the month it appears to repeat the months; i.e., rather than a series of from-to date pairs that make sense, I get the same date pairs repeated. All I can think of is that I'm doing something in the start and end count calculations that's only an issue on the last day of a month.
Results expected are:
From 2013-10-01 to 2013-10-31
From 2013-11-01 to 2013-11-30
...
From 2016-09-01 to 2016-09-30
The results I get when running the report on the last day of the month are a bit random. Here are the dates produced on 31/05 (note that only the TO date is displayed for simplicity. I know the FROM date is the first of the relevant month because the report data is correct).
31/10/2013
31/10/2013
31/12/2013
31/12/2013
31/01/2014
31/03/2014
31/03/2014
31/05/2014
31/05/2014
31/07/2014
31/07/2014
31/08/2014
31/10/2014
31/10/2014
31/12/2014
31/12/2014
31/01/2015
31/03/2015
31/03/2015
31/05/2015
31/05/2015
31/07/2015
31/07/2015
31/08/2015
31/10/2015
31/10/2015
31/12/2015
31/12/2015
31/01/2016
31/03/2016
31/03/2016
31/05/2016
31/05/2016
31/07/2016
31/07/2016
31/08/2016
31/10/2016
Here's the code:
<?php
$reportBegin = new DateTime("2013-10-01"); // start of report
$reportEnd = new DateTime("2016-09-01"); // end of report
$nowRef = new DateTime();
$startCount = (($nowRef->diff($reportBegin)->m) + ($nowRef->diff($reportBegin)->y*12)) * -1; // int * -1 to make neg
$endCount = (($nowRef->diff($reportEnd)->m) + ($nowRef->diff($reportEnd) ->y*12)) + 1; // int and add 1
$count = $startCount;
// do all months
while($count <= $endCount){
$from = date('Y-m-1', strtotime("$count months"));
$to = date('Y-m-t', strtotime($from));
print ("From $from to $to<br />");
$count++;
} // done all months
?>
Can anyone give me a steer? I'm not really sure how to test it.
Edit: The reason I'm mixing DateTime() and date() is because in production, there's a section of code that in some environments replaces the DateTime() section. This replacement code sets $startCount and $endCount as integers.

Answer 1

Here's a PHP5.3 version of the answer in my comment:
<?php
$reportBegin = new DateTime("2013-10-01"); // start of report
$reportEnd = new DateTime("2016-09-01");
$reportEnd->add(new DateInterval('P1M')); // end of report
$interval = new DateInterval('P1M');
$period = new DatePeriod($reportBegin, $interval, $reportEnd);
foreach ($period as $month) {
printf("From %s to %s<br />",
$month->format('Y-m-01'),
$month->format('Y-m-t')
);
}
?>
Demo
PHP 5.4+
<?php
$reportBegin = new DateTime("2013-10-01"); // start of report
$reportEnd = (new DateTime("2016-09-01"))->add(new DateInterval('P1M')); // end of report
$interval = new DateInterval('P1M');
$period = new DatePeriod($reportBegin, $interval, $reportEnd);
foreach ($period as $month) {
printf("From %s to %s<br />",
$month->format('Y-m-01'),
$month->format('Y-m-t')
);
}
?>
Demo