PHP实现按月拆分上下半月并显示英文月份的问题排查

需求背景 #

为薪资系统实现按月拆分显示逻辑：将每个月分为前15天和15号之后的剩余天数，且月份需以英文单词形式展示（如1对应January）。

问题现象 #

初始代码可正常输出数字格式的月份日期范围，但修改为英文月份后，所有月份均显示为January，无法得到对应月份的英文名称：

• 期望输出：

January 1, 2024 <=> January 15, 2024
January 16, 2024 <=> January 31, 2024
February 1, 2024 <=> February 15, 2024
...

• 实际输出：

January 1, 2024 <=> January 15, 2024
January 16, 2024 <=> January 31, 2024
January 1, 2024 <=> January 15, 2024
...

错误代码 #

for($i=1;$i<=12;$i++) {

$totaldays=cal_days_in_month(CAL_GREGORIAN,$i,$currentYear);

$firstDay = date($i.'-01-'.$currentYear);
$firstDay = date('F 1, Y', strtotime($firstDay));

$first15 = date($i.'-15-'.$currentYear);
$first15 = date('F 15, Y', strtotime($first15));
                    
$after15days = date($i.'-16-'.$currentYear);
$after15days = date('F 16, Y', strtotime($after15days));
                    
$lastDay = date($i.'-'.$totaldays.'-'.$currentYear);
$lastDay = date('F '.$totaldays.', Y', strtotime($lastDay));

}

问题原因 #

核心问题出在日期格式的解析歧义：
代码中构造的日期字符串是$i.'-01-'.$currentYear（如2-01-2024），这种“月-日-年”格式并非PHP strtotime()函数默认的标准格式，函数会将其误判为“日-月-年”。例如2-01-2024会被解析成2024年1月2日，因此后续用date('F')获取月份时始终返回January。

解决方法 #

方法1：使用标准日期格式构造字符串 #

将日期格式改为年-月-日（ISO标准格式），避免strtotime()解析错误：

$currentYear = date('Y', strtotime($date_now));

for($i=1;$i<=12;$i++) {
    $totaldays = cal_days_in_month(CAL_GREGORIAN, $i, $currentYear);
    
    // 用年-月-日格式构造日期字符串
    $firstDay = date('F 1, Y', strtotime("{$currentYear}-{$i}-01"));
    $first15 = date('F 15, Y', strtotime("{$currentYear}-{$i}-15"));
    $after15days = date('F 16, Y', strtotime("{$currentYear}-{$i}-16"));
    $lastDay = date('F '.$totaldays.', Y', strtotime("{$currentYear}-{$i}-{$totaldays}"));
    
    // 输出结果
    echo "{$firstDay} <=> {$first15}\n";
    echo "{$after15days} <=> {$lastDay}\n";
}

方法2：使用DateTime类（更可靠） #

DateTime类对日期格式的处理更明确，避免解析歧义：

$currentYear = date('Y', strtotime($date_now));

for($i=1;$i<=12;$i++) {
    $totaldays = cal_days_in_month(CAL_GREGORIAN, $i, $currentYear);
    
    $dateObj = DateTime::createFromFormat('Y-n-d', "{$currentYear}-{$i}-01");
    $firstDay = $dateObj->format('F 1, Y');
    
    $dateObj->setDate($currentYear, $i, 15);
    $first15 = $dateObj->format('F 15, Y');
    
    $dateObj->setDate($currentYear, $i, 16);
    $after15days = $dateObj->format('F 16, Y');
    
    $dateObj->setDate($currentYear, $i, $totaldays);
    $lastDay = $dateObj->format('F '.$totaldays.', Y');
    
    // 输出结果
    echo "{$firstDay} <=> {$first15}\n";
    echo "{$after15days} <=> {$lastDay}\n";
}

最终效果 #

修正后代码可正确输出各月份的英文名称，符合需求：

January 1, 2024 <=> January 15, 2024
January 16, 2024 <=> January 31, 2024
February 1, 2024 <=> February 15, 2024
February 16, 2024 <=> February 29, 2024
...

内容的提问来源于stack exchange，提问作者WP Expert