php如何模拟ping命令：php内置函数exce调用ping命令

<?php
$to_ping='www.jb200.com';
$count=2;
$psize=66;
echo "正在执行php ping命令，请等待...n<br><br>";
flush();
while(1){
echo "<pre>";
exec("ping -c $count -s $psize $to_ping", $list);
for($i=0;$i<count($list);$i++){
print $list[$i]."n";
}
echo "</pre>";
flush();
sleep(3);
}