PDO UPDATE查询实际上不会更新记录

This is my code. I created a code that was a update of news But at moment not show errors. But does not make a update....

 <?php

    require("common.php"); 
    global $host, $dbname, $username, $password, $options; 


    $conteudox = $_POST['conteudo'];

    //$imagem = $_['imagem'];

        if(isset($_POST['conteudo']))
        {

            $dbh = new PDO("mysql:host={$host};dbname={$dbname};charset=utf8", $username, $password);
            $sql = "UPDATE news SET conteudo = '{$conteudox}' WHERE id = '{$id_cont}'";
            $count = $dbh->exec($sql);

                echo "ssssss";

            $dbh = null;
        }
        else
            {
                echo "nnnnn";
            }


?>

try again with

if(isset($_POST['conteudo']))
{
    $dbh = new PDO("mysql:host={$host};dbname={$dbname};charset=utf8", $username, $password);
    $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $stmt = $dbh->prepare("UPDATE news SET conteudo = ? WHERE id = ?");

    // where's $id_cont comming from?
    $count = $stmt->execute(array($_POST['conteudo'],$id_cont));

    echo "ssssss";

    $dbh = null;
}
else
{
    echo "nnnnn";
}

Check all your variables and try to use :

$sql = "UPDATE news SET conteudo = :conteudox WHERE id = :id_cont";

$dbh->prepare($sql);

$count=$dbh->execute(array(':conteudox'=>$conteudox,
                           ':id_cont'=>$id_cont));