PHP日历的算法

<?php
if (function_exists('date_default_timezone_set')) {
    date_default_timezone_set('Asia/Chongqing');
}
$date = isset($_GET['date']) ? $_GET['date'] : date('Y-m-d');
$date = getdate(strtotime($date));
$end = getdate(mktime(0, 0, 0, $date['mon'] + 1, 1, $date['year']) - 1);
$start = getdate(mktime(0, 0, 0, $date['mon'], 1, $date['year']));
$pre = date('Y-m-d', $start[0] - 1);
$next = date('Y-m-d', $end[0] + 86400);
$html = '<table border="1">';
$html .= '<tr>';
$html .= '<td><a href="' . $PHP_SELF . '?date=' . $pre . '">-</a></td>';
$html .= '<td colspan="5">' . $date['year'] . ';' . $date['month'] . '</td>';
$html .= '<td><a href="' . $PHP_SELF . '?date=' . $next . '">+</a></td>';
$html .= '</tr>';
$arr_tpl = array(0 => '', 1 => '', 2 => '', 3 => '', 4 => '', 5 => '', 6 => '');
$date_arr = array();
$j = 0;
for ($i = 0; $i < $end['mday']; $i++) {
    if (!isset($date_arr[$j])) {
        $date_arr[$j] = $arr_tpl;
    }
    $date_arr[$j][($i+$start['wday'])%7] = $i+1;
    if ($date_arr[$j][6]) {
        $j++;
    }
}
foreach ($date_arr as $value) {
    $html .= '<tr>';
    foreach ($value as $v) {
        if ($v) {
            if ($v == $date['mday']) {
                $html .= '<td><b>' . $v . '</b></td>';
            } else {
                $html .= '<td>' . $v . '</td>';
            }
        } else {
            $html .= '<td> </td>';
        }
    }
    $html .= '</tr>';
}
$html .= '</table>';
echo $html;
?>