mysqli_affected_rows($conn) always returns -1, why?

Question

<?php

include("mysql.php");

$errmsg='';

$name='';

$message='';

if(!empty($_POST['name'])&&!empty($_POST['message'])){

$name=$_POST['name'];

$message=$_POST['message'];

echo $name;

echo $message;

}

else {

$errmsg.='You forgot to enter your name<br>';

}

date_default_timezone_set(' Asia/Taipei');

if($errmsg==''){

$stmt=mysqli_prepare($conn,"INSERT guestbook('Name','Message content',' Date and time') VLAUES(?,?,?)");

$now=date("Y-m-d H:i:s");

mysqli_stmt_bind_param($stmt,'sss' ,$name,$message,$now);

mysqli_stmt_execute($stmt);

if(mysqli_affected_rows($conn)>0){

echo 'has Successfully added a message<br/>';

}

else{

echo 'Unable to add a message<br/>';

}

}

else{

echo $errmsg.'Please press the browser's previous page button to re-enter<br/>';

}

echo '<p><a href="liuyan.php">Return to message board</a>< /p>';

?>

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