How to dynamically get date interval between date ranges - php

Question

I am trying make the interval dynamic based on the date range.
Consider this dataset:
$data = [
"2016-06-01": 2,
"2016-06-01": 2,
"2016-06-02": 2,
"2016-06-02": 2,
"2016-06-03": 2,
"2016-06-03": 2,
"2016-06-04": 2,
"2016-06-04": 2,
...
];
I want to sum the values based on date range intervals.
So for eg:
$start_date = "2016-06-01";
$end_date = "2016-07-01";
// some function that returns interval dates based on range
$interval_dates = [
"2016-06-01",
"2016-06-05",
"2016-06-10,
...
];
Based on that I iterate through the $data array and get the sums for intervals.
The caveat is however that the interval dates should be based on the date range:
for eg:
if $end_date was next year, the interval dates would look like this:
$interval_dates = [
"2016-06-01",
"2016-08-01",
"2016-10-01"
];
So the interval is based on the difference between start and end date. The bigger the difference the bigger the intervals.
So far:
public function getIntervalDates($start_date, $end_date, $interval = 5)
{
$start_date = new \DateTime($start_date);
$end_date = new \DateTime($end_date);
// get the difference in days
$diff_object = $start_date->diff($end_date);
$diff_days = $diff_object->days;
// set the interval days based on difference in days
// and the interval
$interval_days = round(intval($diff_days) / $interval);
// set the first date to given start date
$interval_dates[] = $start_date->format("Y-m-d");
$current_date = $start_date;
for($i=0; $i < $interval; $i++) {
// set interval dates by incrementing
// by interval days
$current_date->add(new \DateInterval("P".$interval_days."D"));
// Dont go beyond end date
if($current_date < $end_date) {
array_push($interval_dates, $current_date->format("Y-m-d"));
}
}
// add end date
$interval_dates[] = $end_date->format("Y-m-d");
return $interval_dates;
}
This solution kinda works but is not reliable.
for eg: if choosing a big range eg: 2016-06-10 TO 2020-06-10, it only returns 2 dates.
I think there must be a mathematical formula that was made to solve precisely this kind of a problem.