How to add minutes value in for loop PHP?

I have these for loop code that loop time and increase minute by 20+ minutes.

But I want to include also the starting time. Somehow it’s missing in the output.

My current code :

$slot_count = 5;

$start = strtotime("08:00");

for( $i=0; $i<$slot_count; $i++ )
{
    $start  = $start;
    $end_new = date("H:i:s", strtotime('+20 minutes', $start));  
    $start  = strtotime($end_new);

    echo ''.$end_new.'<br/>';
    
} 

Current output :

08:20:00
08:40:00
09:00:00
09:20:00
09:40:00

Expected output :

08:00:00
08:20:00
08:40:00
09:00:00
09:20:00
09:40:00