$start= "20:00";
$end = "22:00";
$int = 15;


$int *= 60;
$start = strtotime($start);
$end = strtotime($end);

$range = range($start, $end, $int);
$range = array_map(function ($val) { return date("H:i", $val); }, $range);
Var_dump($range);

<?php

$begin = new DateTimeImmutable('19:00');
$end = new DateTimeImmutable('24:30'); // Last 30 min will not be inclued (see output)
$interval = new DateInterval('PT30M'); // Interval 30 min

$daterange = new DatePeriod($begin, $interval ,$end);

$ranges = [];
foreach ($daterange as $key => $range) {
    $ranges[$key] = $range->format('H:i');
    if($key>0) {
        $ranges[$key-1] .= ' - '.$range->format('H:i');
    }
}
array_pop($ranges);

print_r($ranges);

Array
(
    [0] => 19:00 - 19:30
    [1] => 19:30 - 20:00
    [2] => 20:00 - 20:30
    [3] => 20:30 - 21:00
    [4] => 21:00 - 21:30
    [5] => 21:30 - 22:00
    [6] => 22:00 - 22:30
    [7] => 22:30 - 23:00
    [8] => 23:00 - 23:30
    [9] => 23:30 - 00:00
)