how to write PHPUnit test for the following time slot function?

How to write php unit test. The code is working properly but as I am new to php unit testing need help to generate a php unit test for it.

<?php
$duration = 60;
$cleanup = 0;
$start = "10:00";
$end = "18:00";

  function timeslots($duration,$cleanup,$start,$end){
    $start = new DateTime($start);
    $end = new DateTime($end);
    $interval = new DateInterval("PT".$duration."M");
    $cleanupInterval = new DateInterval("PT".$cleanup."M");
    $slots = array();
    for($intStart = $start;$intStart<$end;$intStart->add($interval)->add($cleanupInterval)){
      $endPeriod = clone $intStart;
      $endPeriod->add($interval);
      if($endPeriod>$end){
        break;
      }
      $slots[] = $intStart->format("H:iA")."-".$endPeriod->format("H:iA");
    }
    return $slots;
  }
?>