https://twitter.com/PET_HAL/status/1093117885151469568

try.php

<?php

function convert_date_into_timestamp($date) {
    if (!$date) {
        return null;
    }
    return strtotime($date);
}

$list = array();
$tmp = array(
    'start_date' => '2019/01/21',
    'end_date' => '2019/02/28',
    'name' => 'A'
);
$list[] = $tmp;
$tmp = array(
    'start_date' => '2019/02/22',
    'end_date' => null,
    'name' => 'B'
);
$list[] = $tmp;

$start_date_list = array();
$end_date_list = array();
$tmp = array();

$list_count = count($list);
for ($i = 0; $i < $list_count; $i++) {
    $info = $list[$i];

    $start_date = convert_date_into_timestamp($info['start_date']);
    if ($start_date) {
        $start_date_list[] = $start_date;
    }

    $end_date = convert_date_into_timestamp($info['end_date']);
    if ($end_date) {
        $end_date_list[] = $end_date;
    }

    $arr = array(
        'start_date' => $start_date,
        'end_date' => $end_date,
        'name' => $info['name']
    );
    $tmp[] = $arr;
}

$start_date_list = array_unique($start_date_list);
$start_date_list = array_values($start_date_list);
sort($start_date_list);

$start_date_count = count($start_date_list);
for ($i = 0; $i < $start_date_count; $i++) {
    if ($i === 0) {
        continue;
    }
    $start_date_timestamp = $start_date_list[$i];
    $end_date_list[] = strtotime('-1 day', $start_date_timestamp);
}

$end_date_list = array_unique($end_date_list);
$end_date_list = array_values($end_date_list);
sort($end_date_list);

$end_date_timestamp = array_key_last($end_date_list);
if ($end_date_timestamp) {
    $start_date_list[] = strtotime('+1 day', $end_date_timestamp);
}

$start_date_list = array_unique($start_date_list);
$start_date_list = array_values($start_date_list);
sort($start_date_list);

$data = array();
$start_date_count = count($start_date_list);
for ($i = 0; $i < $start_date_count; $i++) {
    $start_date_timestamp = $start_date_list[$i];
    $end_date_timestamp = $end_date_list[$i];

    $name = '';
    for ($j = 0; $j < $list_count; $j++) {
        $info = $tmp[$j];

        $use_start_date_timestamp = $info['start_date'];
        $use_end_date_timestamp = $info['end_date'];
        if ($use_start_date_timestamp <= $start_date_timestamp
            && ($use_end_date_timestamp == null || ($end_date_timestamp != null && $use_end_date_timestamp >= $end_date_timestamp))) {
            if (!empty($name)) {
                $name .= '、';
            }
            $name .= $info['name'];
        }
    }

    // 2019/04/24 追加(バグ対応：終了日 < 本日 の場合に空行が出力されてしまう)
    if (empty($name)) {
        continue;
    }

    $arr = array(
        'start_date' => (string) date('Y-m-d', $start_date_timestamp),
        'end_date' => $end_date_timestamp != null ? (string) date('Y-m-d', $end_date_timestamp) : '',
        'name' => $name
    );
    $data[] = $arr;
}

var_dump(data);