Hi,
i'm doing a page where user can register and give up of a course:

<td width="5%">
                </td>
                <td>
                PROGRAMAÇÃO:
                <br/>
                <?PHP
                        $cc=$_GET['cod'];
                        $sql_prog = "SELECT prog_curso FROM cursos WHERE cod_curso=".$cc;
               
                        $query_prog = mysql_query($sql_prog, $connect);
               
                        while ($row = mysql_fetch_array($query_prog))
                        {
                                echo $row['prog_curso'];
                        }
                ?>
                <br/>
                </td>
                <?PHP
                if(is_logged())
                {
                ?>
                <form action="inscreve.php?cod=<?php echo $cc ?>" method="post" class="registration_form">
                        <input type="submit" name='inscreve' value="Inscrever" />
                </form>
                <form action="desiste.php?cod=<?php echo $cc ?>" method="post" class="registration_form">
                        <input type="submit" name='desiste' value="Desistir">
                </form>
                <?PHP
                ;}
                ?>
        </tr>

desiste.php:

<?php

include 'topo.php';
include ('db_connect.php');

$cc = $_GET['cod'];
$aluno=$_SESSION['cod_aluno'];

$query_delete_user = "DELETE FROM al_curso WHERE cod_curso=". $cc ." AND cod_aluno=". $aluno;
$result_delete_user = mysql_query($query_delete_user, $connect);

if (!$result_delete_user){
        echo '<div class="warning"> Não estava inscrito neste curso. </div>';
}
else
{
        header("location: desiste_suc.php?cod=$cc");
}

include 'rodape.php';
?>

desiste_suc.php:

<?PHP
        include 'topo.php';
        echo '<div class="success"> O aluno desitiu. </div>';
        include 'rodape.php';
?>

So, the register part it's working perfectly, but the give up part, i have a student register in Guitar and if i give up it Delets ('til here we're ok) now if i give up again it should say "The student is not register in the course", instead of that it says "Give up successfuly".

Can someone help me?

There is an actual PHP forum on this site where this might get answered better...

It might be me, but I don't completely understand what your question is. Or, better said, I understand your question, but the context is kind of unclear to me.

mysql_query returns true even if there are no records deleted. Have a look at mysql_affected_rows. Use that to determine if there are either zero, or one or more rows deleted.