php while循环 指定显示内容 例如不想显示前10条和后10条

<?php

//查询信息总的条数
$db_num = query_num("表","where 1=1");
//每页显示的条数
$page_size=200;
//总条目数
$nums=$db_num;
//每次显示的页数
$sub_pages=5;

if(!$pageCurrent) $pageCurrent=1;

$page_num=$pageCurrent-1;
$page_num=$page_num*$page_size;

$list_sql = mysql_query('SELECT * FROM 表 order by createdtime desc LIMIT ' .$page_num.',' . $page_size);
$xh=0;
$b=$_GET['b'];

while($value=mysql_fetch_array($list_sql)){

$xh=$xh+1;
if( ($b == 1) && ($xh > 51)){
exit("黑化肥挥发发灰会花飞;灰化肥挥发发黑会飞花 。");
}elseif( ($b == 2) && ($xh > 101)){
exit("黑化肥挥发发灰会花飞;灰化肥挥发发黑会飞花 。");
}elseif( ($b == 3) && ($xh > 151)){
exit("黑化肥挥发发灰会花飞;灰化肥挥发发黑会飞花 。");
}elseif( ($b == 4) && ($xh == 199)){
echo("黑化肥挥发发灰会花飞;灰化肥挥发发黑会飞花 。");
}

?>

<?php
if( ($b == 2) && ($xh <= 50)){
echo "<!-- ";
}elseif( ($b == 3) && ($xh <= 100)){
echo "<!-- ";
}elseif( ($b == 4) && ($xh <= 150)){
echo "<!-- ";
}
?>
<tr class="qqlist_a">
<td ><?php echo $value['传真'];?>，<?php echo $value['再传真'];?>，<?php echo $value['tel3'];?></td>
<td ><?php echo $value['扣扣'];?>，<?php echo $value['围脖'];?></td>
<td width="30"><?php if($value[Sex]==1){echo '男';}else{echo '女';}?> </td>
<td ><?php echo $value['电话话'];?>，<?php echo $value['telphone'];?>，<?php echo $value['城市'];?></td>
<td width="60"><?php echo $value['名字'];?></td>
<td width="60"><?php echo $value['记录'];?></td>
<td title="<?php echo clean_xss($value['记录']);?>"><?php echo $xh;?><?php echo mysubstr(clean_xss($value['记录']),10,50);?></td>
</tr>
<?php
echo " -->";
?>

<?php
}
?>