PHP 补全日期区间中缺少的日期/返回缺少的日期

<?php

/**
 * PHP 补全日期区间中缺少的日期/返回缺少的日期
 */

$lists = $list = array();
$result = $db->query("SELECT id, date FROM `table_name` WHERE `areaname` = 'xxx'");

while ($r = $db->fetch_array($result)) {
    $list[] = $r['date'];
    $lists[] = $r;
}

# 补全日期
$newData = spd('2012-01-01', '2013-08-08');
# 缺少的日期
$difference_set_arr = array_diff($newData, $list);
echo '<pre>';
var_dump("'" . implode("','", $difference_set_arr) . "'");
die;

function spd($minimum_time, $maximum_time)
{
    # 日期必须使用字符串类型 因为日期不是数值类型
    $collection = [];
    // 循环计算
    $minimum_time = strtotime($minimum_time);
    $maximum_time = strtotime($maximum_time);
    while ($minimum_time <= $maximum_time) {
        $collection[] += $minimum_time;
        $minimum_time = strtotime('+1 day', $minimum_time);
    }
    foreach ($collection as &$value) {
        $value = date('Y-m-d', $value);
    }
    return $collection;
}