$con= newPDO("mysql:host=localhost;dbname=test",'root','');

$start= 1;

$rows= 5;

$sql= "select * from users limit :start,:rows";

$sth= $con->prepare($sql);

$sth->execute(array(':start'=>$start,':rows'=>$rows));

$arr= $sth->fetchAll();

print_r($arr);

$con= null;

请问大神这个是哪儿出问题了