Problema con el refresh o F5

[Estupefactika]

He probado a meterlo en el constructor de mi página principal que es paginaclases.php, pero me salen estos errores:

Warning: Cannot modify header information - headers already sent by (output started at C:\web\Harvesters\librovisitas.php:24) in C:\web\Harvesters\paginaclases.php on line 9

Warning: Cannot modify header information - headers already sent by (output started at C:\web\Harvesters\librovisitas.php:24) in C:\web\Harvesters\paginaclases.php on line 10

Warning: Cannot modify header information - headers already sent by (output started at C:\web\Harvesters\librovisitas.php:24) in C:\web\Harvesters\paginaclases.php on line 11


Lo que tengo en librovisitas.php es:
<?php


class librovisitas {

function librovisitas (&$obj_harvesters) {
$this->obj_harvesters=$obj_harvesters;
}

function foro_mensajes_leer () {
//Ejecutamos la consulta
$sql="Select * from librovisitas order by id desc";
$libro=$this->obj_harvesters->get_query($sql);

$this->obj_harvesters->smarty->assign("LIBRO",$libro);

}

}


?>