Complete code to implement perpetual calendar in php

<?php
//修改页面编码
header("content-type:text/html;charset=utf-8");

//获取当前年
$year=$_GET[&#39;y&#39;]?$_GET[&#39;y&#39;]:date(&#39;Y&#39;);

//获取当年月
$month=$_GET[&#39;m&#39;]?$_GET[&#39;m&#39;]:date(&#39;m&#39;);

//获取当前月多少天
$days=date(&#39;t&#39;,strtotime("{$year}-{$month}-1"));

//当前一号周几
$week=date(&#39;w&#39;,strtotime("{$year}-{$month}-1"));

//居中
echo "<center>";

//计算上个月
if($month==1)
{
    $prevyear=$year-1;
    $prevmonth=12;
}
else
{
    $prevyear=$year;
    $prevmonth=$month-1;
}

//计算下个月
if($month==12)
{
    $nextyear=$year+1;
    $nextmonth=1;
}
else
{ 
    $nextyear=$year;
    $nextmonth=$month+1;
}

//输出表头
echo " <h2><a href=&#39;万年历.php?y={$prevyear}&m={$prevmonth}&#39;>上一月</a>|{$year}年{$month}月|<a  href=&#39;万年历.php?y={$nextyear}&m={$nextmonth}&#39;>下一月</a></h2>";

//输出日期表格
echo "<table width=&#39;700px&#39; border=&#39;1px&#39;>";
echo "<tr>";
echo "<th>周日</th>";
echo "<th>周一</th>";
echo "<th>周二</th>";
echo "<th>周三</th>";
echo "<th>周四</th>";
echo "<th>周五</th>";
echo "<th>周六</th>";
echo "</tr>";

//铺表格
for ($i=1-$week; $i <=$days ;)
     { 
        echo "<tr>";
        for ($j=0; $j < 7; $j++)
         { 
            if ($i>$days || $i<1) 
            {
                echo "<td> </td>";
            }
            else
            {
                 echo "<td>{$i}</td>";
            }
            $i++;
        }
        echo "</tr>";
    }
echo "</table>";
echo "</center>";

?>