MySQL查询到PDO

I'm switching from MySQL to PDO and I'm unsure if this query is correct.. would I still be required to write the if command.

public function User_Login($_iUsername,$_iPassword) {
    $username=mysql_real_escape_string($_iUsername);
    $password=mysql_real_escape_string($password);
    $md5_password=md5($_iPassword);
    $query=mysql_query("SELECT _iD FROM users WHERE _iUsername='$_iUsername' and _iPassword='$md5_password' AND _iStatus='1'");
    if( mysql_num_rows( $query ) == 1 ) {
        $row = mysql_fetch_array( $query );
        return $row['_iD'];
    } else {
        return false;
    }
}

TO

public function User_Login($_iUsername,$_iPassword) {
    $md5_password = md5($_iPassword);
    $sth = $db->prepare("SELECT _iD FROM users WHERE _iUsername='$_iUsername' and _iPassword='$md5_password' AND _iStatus='1'");
    $sth->execute();

    $result = $sth->fetchAll();
}

First off, you're not properly parameterizing the query. It's great that you're using PDO, but one of the main purposes of the change is the ability to parameterize queries. Secondly, md5 is a very weak hash. I suggest using bcrypt instead. Finally, PDOStatement::rowCount is the method you are looking for.

$sth = $db->prepare("SELECT _ID FROM users WHERE _iUsername = ?
    AND _iPassword = ? AND _iStatus = 1");
$sth->execute(array($_iUsername, $md5_password));
if ($sth->rowCount() == 1) {
    $row = $sth->fetch(PDO::FETCH_ASSOC);
    return $row['_iD'];
}
else {
    return false;
}