Why does my datetime have trouble with the 31st? - php

Question

I have the following piece of code on a site I'm working on. It was written by a previous developer, so I am unsure of exactly how to deal with it:
$datetime = new DateTime("$dt_start_ymd +$gap month");
$dt_next = new DateTime("Thursday " . $datetime->format('Y-m-01'));
$dt_next_0 = $dt_next->setTime(02, 00)->getTimestamp();
$dt_next = $dt_next->getTimestamp();
The variable $gap is an incrementing count (1, 2, 3, etc). The code above should output a sequence of dates listing the first thursday for each month for a number of months as indicated by $gap
i.e. 05/07/2017, 03/08/2017, 07/09/2017 (those being the first Thursday of those respective months).
The code works fine on most dates, but for a reason I cannot fathom if it is passed a date that is the 31st of a month (i.e. 2017-05-31) the system breaks and outputs incorrect answers.

Answer 1

I think this code is unnecessarily complicated. In addition, you're in a situation where if the given date is the 31st and you attempt to add 1 month, you're ending up in a situation where it's attempting to find the 31st of the next month, which does not exist except in December->January.
You might want to consider something like
$currentDate = new \DateTime();
$iterations = 10;
for ($i = 0; $i < $iterations; $i++) {
$first_thursday = new \DateTime(
sprintf('first Thursday of %s', $currentDate->format('F Y'))
);
$nextDate = new \DateTime($currentDate->format('Y-m-d'));
$nextDate->add(new \DateInterval('P1M'));
if ($nextDate->format('m') > $currentDate->format('m') + 1) {
$nextDate->sub(new \DateInterval('P1D'));
}
$currentDate = $nextDate;
echo $first_thursday->format('Y-m-d');
}
This will give you the first Thursday in every month for the next 10 months, and I think is much easier to read than the example you gave.
EDIT: I've updated the code so that it doesn't skip out months. See my comment below.