pdo mysql怎么输出第1条 第4条 第7条数据

<?php  $pdo = new PDO("mysql:host=localhost;dbname=uzhan","root","");  $sql1="select * from www"; $rs = $pdo -> query($sql1);  while($row = $rs -> fetch()) {  echo'<div>'.$row[id].'</div>'; } ?>

<?php  $pdo = new PDO("mysql:host=localhost;dbname=uzhan","root","");  $sql1="select * from www"; $rs = $pdo -> query($sql1); $num = 0; while($row = $rs -> fetch())  { $num++;if(1==$num || 4==$num || 7==$num)   echo'<div>'.$row[id].'</div>';  }

<?php$pdo = new PDO("mysql:host=localhost;dbname=uzhan","root","");$sql1="select * from www";$rs = $pdo -> query($sql1);$flag = 1;while($row = $rs -> fetch()){  if(($flag-1)%3 == 0){    echo'<div>'.$row[id].'</div>';  }  $flag++;}

  $pdo = new PDO("mysql:host=localhost;dbname=uzhan","root","");  $sql1="select * from www"; $rs = $pdo -> query($sql1);  while($row = $rs -> fetch()){ $i = 1;if($i % 3 ==1){ echo'<div>'.$row[id].'</div>';  }  $i = $i + 1;}