week_streak.php

<?php

$dates = [
      "12/01/2021",
      "12/05/2021",
      "12/09/2021",
      "12/10/2021",
      "12/11/2021",
      "12/15/2021",
      "12/10/2021",
      "12/31/2021",
      "01/10/2022",
      "01/17/2022",
      "01/23/2022",
    ];

$dates_to_timestamps = [];
foreach ($dates as $d) {
  $v = DateTimePlus::createFromFormat("m/d/Y", $d);
  $dates_to_timestamps[] = $v->getTimestamp();
}
sort($dates_to_timestamps);

$one_week = 7 * 24 * 60 * 60;
$streak = 0;

$week_start = reset($dates_to_timestamps) + $one_week;
$week_end = $week_start + $one_week;

foreach ($dates_to_timestamps as $t) {
  if ($t < $week_start) {
    continue;
  }

  if ($t >= $week_start && $t <= $week_end) {
    $streak++;
    $week_start = $week_end;
    $week_end = $week_end + $one_week;
  }
  else {
    $streak = 0;
    if ($t >= $week_end) {
      $week_start = $t + $one_week;
      $week_end = $week_start + $one_week;
    }
  }
}

return $streak;