php replacing old code with DateTime class - php

Question

I have a function that creates time intervals between two time marks. The function works but I'm struggling to upgrade from strtotime() and use the DateTime class.
Below is a patch of code I wrote without getting errors
$timestart = new DateTime("14:00:00");
$timestop = new DateTime("20:00:00");
$date_diff = $timestop->diff($timestart);
$time_diff = $date_diff->format('%H');
Next is the entire code untouched. I get DateInterval could not be converted to int erros using the code above. Please kindly advise how to correctly implement the class.
Live example: http://codepad.org/jSFUxAnp
function timemarks()
{
//times are actually retrieved from db
$timestart = strtotime("14:00:00");
$timestop = strtotime("20:00:00");
$time_diff = $timestop - $timestart; //time difference
//if time difference equals negative value, it means that $timestop ends second day
if ($time_diff <= 0)
{
$timestop = strtotime( '+1 day' , strtotime( $row->timestop ) ); //add 1 day and change the timetsamp
$time_diff = $timestop - $timestart;
}
//create interval
$split = 3;
$interval = $time_diff/$split;
//get all timemarks
$half_interval = $interval / 2;
$mid = $timestart + $half_interval;
for ( $i = 1; $i < $split; $i ++) {
//round intervals
$round_mid = round($mid / (15 * 60)) * (15 * 60);
$result .= date('H:i:s', $round_mid) . ", ";
$mid += $interval;
}
$round_mid = round($mid / (15 * 60)) * (15 * 60);
$result .= date('H:i:s', $round_mid);
return $result;
}
outputs 15:00:00, 17:00:00, 19:00:00

Answer 1

Actually you're using DateTime, these are just aliases for creating DateTime instances
The equivalent would look like this:
$timestart = new DateTime("14:00:00");
$timestop = new DateTime("20:00:00");
$date_diff = $timestop->diff($timestart);
$time_diff = $date_diff->format('%H');
So this has to work, I tested it and I got correct results!