PHP Hatası Yardım!

<?php $query = $db->query("SELECT * FROM sitekategoriler");$query->execute();
if ( $query->rowCount() ){     
    foreach( $query as $row ){          
        echo $row['kat_adi']."<br />";     
    }
}
?>

<?php

require_once 'baglan.php';

$servername = "localhost";
$database = "demo";
$username = "root";
$password = "roottoor";
// Create connection
$conn = mysqli_connect($servername, $username, $password, $database);
// Check connection
if (!$conn) {
    die("Connection failed: " . 
mysqli_connect_error());
}
mysqli_close($conn);
?>

<?php

require_once 'baglan.php';

$servername = "localhost";
$database = "demo";
$username = "root";
$password = "roottoor";
// Create connection
$conn = mysqli_connect($servername, $username, $password, $database);
// Check connection
if (!$conn) {
    die("Connection failed: " .
mysqli_connect_error());
}
mysqli_close($conn);
?>

<?php $query = $conn->query("SELECT * FROM sitekategoriler");
$query->execute();
if ( $query->rowCount() ){     
    foreach( $query as $row ){          
        echo $row['kat_adi']."<br />";     
    }
}
?>

<?php

    $result = $conn->query("SELECT * FROM sitekategoriler");

    if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {
        echo $row['kat_adi']."<br />";     
    }
    } else {
        echo "0 results";
    }
    $conn->close();

?>

<?php
$mysqlsunucu = "localhost";
$mysqlkullanici = "root";
$mysqlsifre = "roottoor";
try {
    $conn = new PDO("mysql:host=$mysqlsunucu;dbname=demo;charset=utf8", $mysqlkullanici, $mysqlsifre);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }
catch(PDOException $e)
    {
    echo "Bağlantı hatası: " . $e->getMessage();
    }
?>

<?php $query = $db->prepare("SELECT * FROM sitekategoriler");$query->execute();if ( $query->rowCount() ){     foreach( $query as $row ){          echo $row['kat_adi']."<br />";     }}?>