How to set cookies and set expiration time?

Question

I am with a div where I search only logged in users, in it I am displaying users but each update is inserted new records, gave me the tip to use cookise more never worked with it. the following code is giving the error Warning: Cannot Modify header information - headers already sent by (output Started at line 5

<?php
$id_usuario = $_SESSION['user_id'];
 $usuario = $_SESSION['user_name'];

setcookie($usuario, $id_usuario, time() + (86400 * 30), "/");

if(!isset($_COOKIE[$usuario])) {
echo "Cookie named '" . $usuario . "' is not set!";
} else {
echo "Cookie '" . $usuario . "' is set!<br>";
echo "Value is: " . $_COOKIE[$usuario];
}
?>

following this tutorial http://www.w3schools.com/php/php_cookies.asp

Answer 1

On the page where the user logs in, put the following:

setcookie($usuario, $id_usuario, time() + (60 * 10), "/");

That will make it last for 10 minutes. Already in the rest of the page, put this:

<?php
ob_start()
$id_usuario = $_SESSION['user_id'];
$usuario = $_SESSION['user_name'];

if(!isset($_COOKIE[online])) {
include('usuariosonline.txt');
} else { 
//grava os dados no arquivo
setcookie(online, $id_usuario, time() + (60 * 10), "/");
$arquivo = fopen("usuariosonline.txt", "a");
fwrite($arquivo, "<p><img src='avatar/".$id_usuario.".jpg'/>".$usuario."</p>\n");
fclose($arquivo);
include('usuariosonline.txt');
}
?> 

This code will show the users file online if the cookie already exists and if it does not exist, it will automatically create the cookie and write the user name in the users file online