如题求教，请问怎么用循环结构制作日历啊？作业帮帮忙

<?php
$year=$_GET['y']?$_GET['y']:date("Y");
$mon=$_GET['m']?$_GET['m']:date("m");
$day=date("t",mktime(0,0,0,$mon,1,$year));
$w=date("w",mktime(0,0,0,$mon,1,$year));
echo"<center>";
echo"<h1>{$year}年{$mon}月</h1>";
echo"<table width='600', border='1'>";
echo"<tr>";
echo "<th>星期日</th>";
echo "<th>星期一</th>";
echo "<th>星期二</th>";
echo "<th>星期三</th>";
echo "<th>星期四</th>";
echo "<th>星期五</th>";
echo "<th>星期六</th>";
echo "</tr>";
$dd=1;
while($dd<=$day){
echo"<tr>";
for ($i=0 ; $i < 7; $i++) {
if ($dd<=$day&&($w<=$i ||$dd!=1)) {
# code...
echo "<td>{$dd}</td>";
$dd++;
}
# code...
else{echo"<td> </td>";}
}
echo "</tr>";
}
echo "</table>";
$prey=$nexty=$year;
$prem=$nextm=$mon;
if($prem<=1)
{$prem=12;
$prey--;
}
else $prem--;
if($nextm>=12)
{$nextm=1;
$nexty++;
}
else $nextm++;
echo"<a href='data.php?y={$prey}&m={$prem}'>上一月</a>";
echo"<a href='data.php?y={$nexty}&m={$nextm}'>下一月</a>";
echo"</center>";
?>

用了楼上的代码后变成这样子了，要怎么改正呢？