unexpected while-loop behavior - php

Question

I've tried building and placing the while loop in different ways and different locations and looking at $val as the right/wrong variable to be placing in the while loop but I'm just not getting it. I expected output to be: 815 830 845 900....... until reaching 1900.
Instead I got 815 then 1915 1930 1945 up through 0000 then it cycles over starting at 1915. Someone please tell me what i have placed in the wrong location or what variable I've used wrongly.
date_default_timezone_set('America/New_York');
$st='800';
$et='1900';
$frac = 900;
$x = $et;
$val=0;
if (strlen($st) < 4) {$st = "0".$st;}
$current_time = strtotime(date($st));
$r = $current_time % $frac;
$new_time = $current_time + ($frac-$r);
$val = date('Gi', $new_time);
echo $val."<br>";
while ($val !== $et){
if (strlen($val) < 4) {$st = "0".$val;}
$current_time = strtotime('+ 15 minutes',date($val));
$r = $current_time % $frac;
$new_time = $current_time + ($frac-$r);
$val = date('Gi', $new_time);
echo $val."<br>";
}
NOTE TO THE - MARKDOWN PLAYERS -- Edited POST ANSWER: You can bash the question all you want, but the fact is it was a clear question, clear objective trying to be reached and it was a clear attempt (one of several) to code it properly that failed, so bad code, not bad effort. So eventually I asked for help. Your using the markdown system the wrong way and for the wrong reason, but whatever I have little expectation that you care. Thanks to those that made the effort actually be constructive and teach/help.

Answer 1

With unelegant for loop:
for ($i = 800, $t= 1900; $i <= $t; $i += 15) {
printf('%1$3d <br>', $i);
if ($i%100 >= 45) $i += 40;
}
With DatePeriod:
$timezone = new DateTimeZone("America/New_York");
$begin = new DateTime("now", $timezone);
$begin->setTime(8, 0, 0);
$end = new DateTime("now", $timezone);
$end->setTime(19, 0, 0);
$interval = new DateInterval('PT15M');
$end->add($interval);
$daterange = new DatePeriod($begin, $interval ,$end);
foreach($daterange as $date){
echo $date->format("Gi") . "<br>";
}
With generators:
function xmins($start, $end, $step = 15) {
if ($start < $end) {
if (($step <= 0) || ($step >= 40)) {
throw new LogicException('Step must be in range 1-40');
}
for ($i = $start; $i <= $end; $i += $step) {
if (60 <= $temp = $i % 100) {
$i += 40;
}
yield $i;
if ((60-$step) === $temp = $i % 100) {
$i += 100 - $temp - $step;
}
}
}
}
foreach (xmins(800,1900,15) as $i) {
printf('%1$3d<br>', $i);
}

Answer 2

Using DateInterval:
$date = new DateTime('2015-01-01 08:00:00');
$mins = new DateInterval('PT15M');
do {
$date->add($mins);
echo $date->format('Hi')."\n";
} while($date->format('Hi') < 1900);