在关联数组/Map中按年和按月收集日期。现在，分别进入关联数组中的每个月并usort它们。这样，与对整个数据集进行排序并将日期与甚至不属于同一桶的其他日期进行比较相比，排序成本将是最小的！
最后，在map上执行array_values以删除散列索引（例如用作整数的年或月）。

打喷嚏：

<?php

$data = [];

foreach($reserved_dates as $d){
    list($year, $month) = explode("-", $d);
    $data[ $year ]['year'] = $year;
    $data[ $year ]['months'][ $month ] = $data[ $year ]['months'][ $month ] ?? [];
    $data[ $year ]['months'][ $month ]['monthName'] = date("F", strtotime($d));
    $data[ $year ]['months'][ $month ]['paid'] = true; // or false whatever
    $data[ $year ]['months'][ $month ]['dates'] = $data[ $year ]['months'][ $month ]['dates'] ?? [];
    $data[ $year ]['months'][ $month ]['dates'][] = $d;
}

foreach($data as &$months){
    foreach($months['months'] as &$each_month){
        usort($each_month['dates'], fn($a, $b) => DateTime::createFromFormat('!Y-m-d', $a) <=> DateTime::createFromFormat('!Y-m-d', $b));
    }
    $months['months'] = array_values($months['months']);
}

$data = array_values($data);

print_r($data);

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Live Demo

如果你希望月份数组的关键字是数字索引而不是月份名称，那么你可以通过使用array_values函数重新索引月份数组来实现这一点。
此外，在reserved_dates循环中，您从日期数组访问日期，但reserved_dates的值不可用。
请尝试下面的代码。

<?php
    $reserved_dates = [
    "2023-12-03",
    "2023-12-10",
    "2023-12-17",
    "2023-12-24",
    "2023-12-31",
    "2024-01-07",
    "2024-01-14",
    "2024-01-21",
    "2024-01-28",
    "2024-02-04",
    "2024-02-11",
    "2024-02-18",
    "2024-02-25",
    "2024-12-01",
    "2024-12-08",
    "2024-12-15",
];

$years = [];
foreach ($reserved_dates as $date) {
    $d = $date;
    list($y, $m) = explode("-", $d);
    $years[$y][] = $d;
}
$years = array_values($years);

$reservation = ["reservedDates" => []];

foreach ($years as $year) {
    $year_number = date("Y", strtotime($year[0]));

    $new = [
        "year" => $year_number,
        "months" => [],
    ];

    foreach ($year as $month) {
        $month_name = date("F", strtotime($month));

        if (!isset($new["months"][$month_name])) {
            $new["months"][$month_name] = [
                "monthName" => $month_name,
                "dates" => [],
            ];
        }

        array_push($new["months"][$month_name]["dates"], $month);
    }

    // re indexing month array
    $new["months"] = array_values($new["months"]);

    array_push($reservation["reservedDates"], $new);
}

echo "<pre>";
print_r($reservation);
?>

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