Description:
------------
DateTime::add adding 1 month on 31st of August skips to October

I did a similar snippet in Java and I get the expected result:

import java.util.*;
import java.text.*;
import java.lang.*;

class TestingDateTime
{
    public static void main (String[] args) throws java.lang.Exception
    {
        SimpleDateFormat ft = new SimpleDateFormat ("yyyy-MM-dd");
        Date date = ft.parse("2016-08-31");

        System.out.println("Initial value:");
        System.out.println(ft.format(date));

        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.add(Calendar.MONTH, 1);

        date = calendar.getTime();

        System.out.println("\nNew value:");
        System.out.println(ft.format(date));
    }
}


Test script:
---------------
<?php


function printline($value) {
	$separator = 'cli' == php_sapi_name() ? PHP_EOL : '<br/>';
	
	echo $value . $separator;
}


$dateTime = new DateTime('2016-08-31 00:00:00');
printline('Initial value: ' . $dateTime->format('Y-m-d H:i:s'));

$dateTime->add(new DateInterval('P1M'));
printline('New value: ' . $dateTime->format('Y-m-d H:i:s'));

Expected result:
----------------
2016-09-30 00:00:00

Actual result:
--------------
2016-10-01 00:00:00