<?php

function dates() {
  yield new DateTimeImmutable('2020-01-01');
  yield new DateTimeImmutable('2020-01-02');
  yield new DateTimeImmutable('2020-01-05');
  yield new DateTimeImmutable('2020-01-06');
  yield new DateTimeImmutable('2020-01-10');
}

$next = new DateTimeImmutable('2020-01-01');

foreach (dates() as $date) {
  while ($date != $next) {
    echo $next->format('Y-m-d H:i:s') . '<br>';
    $next = $next->add(new DateInterval('P1D'));
  }

  echo $date->format('Y-m-d H:i:s') . '<br>';
  $next = $next->add(new DateInterval('P1D'));
}