PDO probleme avec la methode query

$query="SELECT id_member FROM table_members WHERE login='".$login."'";
$sth=$this->dbh->query($query);
$result=$sth->fetchAll();
$num_rows=count($result);
if($num_rows==0)
{
//.....
puis plus loin:
....//

...//
$query="SELECT id_member FROM table_members WHERE login='".$login."' AND password='".sha1($password)."'";
$sth=$this->dbh->query($query);
$result=$sth->fetchAll();
$num_rows=count($result);
if($num_rows==0)
{
//...

$query="SELECT id_member FROM table_members WHERE login='".$login."' AND password='".sha1($password)."'";
$sth_2=$this->dbh->query($query);
$result=$sth_2->fetchAll();
$num_rows=count($result);
if($num_rows==0)
{

$sth=null;