错误消息“在非对象上调用成员函数查询（）”

Question

In the following code, I am trying to check if the username entered has been used or not by counting the username.

<?php
$hostname = 'aaa';
$username = 'bbb';
$password = 'ccc';
$prefix = "";
$dbna = 'ddd';
$username11 = $_POST['username'];
try {
$dbh = new PDO("mysql:host=$hostname;dbname=$dbna", $username, $password);
$nRows = $pdo->query("SELECT count(Username) FROM Users WHERE Username ='$username11'")->fetchColumn(); 
if($nRows)
{
echo '<span style="color: red;">The username <b>'.$username11.'</b> is already in use.    </span>';
}
else
{
echo 'OK';
}
}
catch(PDOException $e)
{
echo $e->getMessage();
}
?>

The error is:

Fatal error: Call to a member function query() on a non-object in /home/check.php on line 10

I looked for the solution for this error but I couldn't find it.

Answer 1

$pdo->query

should be:

$dbh->query