同时进行PDO SQL查询

Question

i'm having problems with translating simple sql queries to pdo sql queries but my code doesn't seems to run...

I had something like this in simple sql:

Sql(query) 
If(row == variable){
Sql(query)
If(row>variable){
Sql(query)
}
}

This worked in simple sql queries but trying to use pdo doesn't work... I don't know why... Y have to make several pdo for every query? I'm using only one dbh pdo...

require 'dbdata.php';
$fb = $_POST["FB"]; 
$gg = $_POST["GG"];
$points = $_POST["Points"];
$lb = $_POST["leaderboard"];
$ID;
try {
    $DBH = new PDO("mysql:host=$host;dbname=$dbname", $user, $pass);
    if(!empty($fb)){
        $FBQ = $DBH->prepare("SELECT ID FROM Usuarios WHERE FbID='$fb'");
        $count = $FBQ->rowCount();
        $FBQ->setFetchMode(PDO::FETCH_ASSOC);
        while($row = $FBQ->fetch()) {
        $ID = $row['ID'];}
        if($count > 0){
            $LBQ = $DBH->prepare("SELECT * FROM $leaderboard WHERE UserID = $ID");
            $countlb = $LBQ->rowCount();
            $LBQ->setFetchMode(PDO::FETCH_ASSOC);
            if($countlb >0){
                while($row = $LBQ->fetch()) {

                    if($row['Puntuacion'] < $points){
                        $LBQS = $DBH->prepare("UPDATE $leaderboard SET Puntuacion = $points WHERE UserID = $ID");
                        $LBQS -> execute();
                        echo "Actualizado Record";
                    }
                }
            }
        }
    }
}

Any help is apreciated, thanks in advance!