I have no PC to test, but so it should scroll:
<?php
$start = '08:12';
$end = '15:30';
$st = explode(':', $start);
$ed = explode(':', $end);
for($hour = $st[0]; $hour <= $ed[0]; $hour++)
{
for($min = $st[1]; $min < 60; $min += 15)
{
if($ed[1] >= $min and $hour <= $ed[0])
echo "Hour: {$hour}:{$min}<br/>";
}
}
I separate the hour in the colon, and in a time from the start time to the end. Logical if depending on the hours may not happen, for example if the interval is from 23 to 02, but as was not quoted in the question if such a case can occur then I did not treat.