计算PDO准备语句中的行数

I have two questions regarding codes below.

I know the second code is correct but not sure if first is also correct both do same thing first one is just easy to write.
I want to count the number of rows in database for the selected element if ($query->num_rows == 1) { doesn't work so how to rowcount for the code below.

First code:

 $query = $db->prepare("SELECT * from users WHERE username = :username");
 $query->execute(array(':username'=>$un));

Second:

$result = "SELECT * from users WHERE username = :username";
$query = $db->prepare( $result ); 
$stmt->bindValue(':username'=>$un);
$query->execute($stmt);

First, if you want to ensure that only one username is selected, you can use LIMIT in your MySQL statement

SELECT * from users WHERE username = :username ORDER BY id DESC LIMIT 1

Or:

SELECT DISTINCT(username) from users WHERE username = :username```

Even better, when creating the table, you can require that the username is unique:

CREATE TABLE users (
    id INT NOT NULL AUTO_INCREMENT,
    username VARCHAR(255) NOT NULL,
    ...
    PRIMARY KEY (id),
);

Second, to verify that a row was actually retrieved from the dabase, you can use fetch:

$query = $db->prepare("SELECT * from users WHERE username = :username");
$query->execute(array(':username'=>$un));
$rows = $query->fetch(PDO::FETCH_NUM);
if($rows[0]) { 
    // Row exists
}

You don't need the row count. You need just the row itself. So, just fetch it, from the first variant, which is ok.

As of the row count, you are supposed to be able to get the proper function name from manual.

$query = $db->prepare("SELECT 1 from users WHERE username = ?");
$query->execute(array($un));
if ($query->fetch())
{
    // found
}