为什么echo json_encode（$ arr）没有为wp_posts查询显示任何内容？

I want to make json of my wordpress posts . When I do same for wp_users , it works but it does not work for wp_posts .It shows nothing in the browser.Here is my code:

    <?php
    $host="localhost";
    $db_username="root";
    $db_password="";
    $db_name="test";
    $conn=mysqli_connect($host,$db_username,$db_password,$db_name) or die('<h1 style="color:#ff0000;>Error
            Connecting To Database !</h1>');
    $sql="SELECT * FROM wp_posts;";
    $result=mysqli_query($conn,$sql) or die('Error executing query .');
    $arr=array();
    while ($r=mysqli_fetch_assoc($result)) {
        $arr[]=$r;
    }
    echo json_encode($arr);
?>