Error creating COOKIE... Cannot Modify header information

Question

I just picked up a project with a friend and came across the following Cookie mistake:

Warning: Cannot Modify header information - headers already send by (output started at...)

I’ll put the code snippet:

<?php
ini_set('output_buffering','On');
ob_start();
require 'admin/util/Conexao.class.php';
require 'admin/util/protecao.php';

if(!empty($_REQUEST['login']) && !empty($_REQUEST['senha'])){
    $login = $_REQUEST['login'];
    $senha = $_REQUEST['senha'];

    $busca = new Conexao("SELECT * FROM convites WHERE login = '$login' AND senha = '$senha'");
    $rs = $busca->query();
    $total = mysql_num_rows($rs);

    if($total != 1){
        echo '<script type="text/javascript">alert("Login incorreto")</script>';
        echo '<script type="text/javascript">window.location = "login.php"</script>';   
    }
    $row = mysql_fetch_array($rs);
    $busca->desconectar();

    setcookie('id_convite',$row['id_convite'],time()+3600);
    echo '<script type="text/javascript">window.open("formulario.php", "_parent");    </script>';

}elseif(!empty($_REQUEST['cpf']) && !empty($_REQUEST['email'])){
    $cpf = anti_injection($_REQUEST['cpf']);
    $email = anti_injection($_REQUEST['email']);

    $busca = new Conexao("SELECT * FROM convites_alunos WHERE cpf = '$cpf' AND email = '$email' AND confirmado = 'N'");
    $rs = $busca->query();
    $total = mysql_num_rows($rs);
    if($total != 1){
        echo '<script type="text/javascript">alert("Login incorreto")</script>';
        echo '<script type="text/javascript">window.location = "login.php"</script>';   
    }
    $row = mysql_fetch_array($rs);
    $busca->desconectar();

    setcookie('id_aluno',$row['id_aluno'],time()+3600);
    echo '<script type="text/javascript">window.open("formulario_aluno.php", "_parent");</script>';

}else{
    echo '<script type="text/javascript">alert("Login incorreto")</script>';
    echo '<script type="text/javascript">window.location = "login.php"</script>';
}
ob_end_flush();
?>

And the error occurs on the line where the cookie is created:

setcookie('id_convite',$row['id_convite'],time()+3600);

Could you help me??? As you can see I already started the buffer and closed, as I saw on the net, but nothing... I used the command:

ini_set('output_buffering','On');

However, the error persists, I am only dependent on it to deliver the project. Thank you.