$y = date('Y');
$m = date('m');
$t = date('t');
$d = 1;
if(isset($_GET["month"])) {
    $m = $_GET['month'];
}
if(isset($_GET["year"])) {
    $y = $_GET['year'];
}

// 前月へのリンク翌月へのリンクを表示
$now = $y ."年" . $m ."月";
$prev = mktime(0, 0, 0, $m - 1 , 1, $y);
$next = mktime(0, 0, 0, $m + 1 , 1, $y);
echo "<table><thead><tr><td colspan='7'><a href='/dojo?year=".date('Y', $prev)."&month=".date('m', $prev)."'>前月へ</a>" . $now . "<a href='/dojo?year=".date('Y', $next)."&month=".date('m', $next)."'>翌月へ</a></td></tr></thead><tr>";

// 曜日を表示
$week = array( '月', '火', '水', '木', '金', '土', '日' );
for ($i = 1; $i <= 7; $i++) {
    echo "<th>" . $week[$i - 1] . "</th>";
}

echo "</tr><tr>";

// 始まりの曜日に応じて空白を表示
$i = 1;
$startWeek = (date("w", mktime(0, 0, 0, $m, 1, $y)) - 1);
if($startWeek < 0) {
    $startWeek = 6 - ($startWeek) - 1;
}
while ($i <= $startWeek) {
    echo "<td></td>";
    ++$i;
}

// 日付を表示
while(checkdate($m, $d, $y)) {
    echo "<td>" . $d . "</td>";
    $d++;
    // 日曜日で改行
    $timestamp = mktime(0, 0, 0, $m, $d, $y);
    if (date("w", $timestamp) === "1") {
        echo "</tr><tr>";
    }
}
echo "</tr></table>";