Form Variable in PDO Query

A Submit button for a form located on my page triggers the code below, but I am unsure of how to save the numeric value of a textbox named 'amount' into a php variable which I can use in the below PDO query. I am unsure of the syntax. Thanks in advance!

     if ((isset($_POST["MM_update"]))开发者_StackOverflow && ($_POST["MM_update"] == "form1")) {
$amount = isset($_POST['amount']) ? $_POST['amount'] : null;
if (null != $amount) {

$user = 'user';
$pass = 'pass';
$pdo = new PDO('mysql:host=localhost;dbname=dbname', $user, $pass);
session_start();
$tablename = $_SESSION['MM_Username'];
$UpdateQuery = sprintf('UPDATE `%s` SET `stock` = :amount WHERE `status` = 1', $tablename);
$stmt = $pdo->prepare($UpdateQuery);
$stmt->bindParam('amount', $amount);
$stmt->execute();
}
}

Removed db_select line and also have verified that the SESSION variable MM_Username is in fact set properly. Is there anyway SQL can spit back more detailed error reporting? When I run the code as it is above, I receive no errors, however, it simply does not work.

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Edit: Totally revised answer with error reporting via Exception

session_start();
if (isset($_POST['MM_update']) && $_POST['MM_update'] == 'form1') {
    $amount = isset($_POST['amount']) ? $_POST['amount'] : null;
    if (null === $amount) {
        throw new Exception('Amount not set');
    }
    $user = 'user';
    $pass = 'pass';
    if (empty($_SESSION['MM_Username'])) {
        throw new Exception('Username table not set in session');
    }
    $pdo = new PDO('mysql:host=localhost;dbname=dbname', $user, $pass);
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $query = sprintf('UPDATE `%s` SET `stock` = :amount WHERE `status` = 1', $_SESSION['MM_Username']);
    $stmt = $pdo->prepare($query);
    $stmt->bindParam('amount', $amount);
    $stmt->execute();
} else {
    echo 'Nothing to do';
}