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;
 ?>