How do I subtract minutes from hours in PHP using seconds?

I am using the following code to try and subtract minutes from seconds. These are all represented using seconds.

//Hours worked in seconds (8 hrs)
$hours_worked = 28800;
//Break time in seconds (15 mins)
$break_mins = 1500;

$total_hours = round(($hours_worked - $break_mins) / 60 / 60 , 2);

echo $total_hours;

Output: 7.58

I was expecting the output to be "7.75" as 15 minutes = 0.25 of an hour, right?

Am I missing something obvious here? I must be!

I have been trying to figure this out for hours now and cant wrap my head round what I'm doing wrong. Any help would be greatly appreciated.