Código PHP:
<form action="libro.php?id=<?php echo $_GET['id']?>" method="post" />
Nombre:
<br/>
<input type="text" name="user"/>
<br/>
Comentarios:
<br/>
<textarea name="comment"></textarea>
<br/>
<br/>
<input type="text" name="id_nt" value="<?php echo $_GET['id']?>"/>
<input type="submit" value="Enviar" />
</form>
<?PHP
include("connection.php");
mysql_query("INSERT INTO comments (id_nt,user,comment)
VALUES('$_REQUEST[id_nt]','$_REQUEST[user]','$_REQUEST[comment]')",$connection)or die(mysql_error());
?>