Session and session variables not working

[jgbranco]

I have this code for the login.php:

<?PHP
    session_start();
    unset($_SESSION['usuario']);
    unset($_SESSION['senha']);
    session_destroy();
    if(isset($_POST['submit'])){
        $usuario = $_POST['usuario'];
        $senha = $_POST['senha'];
        session_start();
        $_SESSION['usuario'] = $usuario;
        $_SESSION['senha'] = $senha;
        header('location: index.php');
    }
?>

this goes before the form I made witch is:

<form name="form1" method="post" action="<?PHP echo($PHP_SELF); ?>">
<input name="usuario" type="text" class="caixa" id="usuario">
<input name="senha" type="text" class="caixa" id="senha" maxlength="6">
<input name="submit" type="submit" class="botao" value="Enviar">
</form>

then in each page of the admin I check if the user is logged:

<?PHP
    session_start();
    if(isset($_SESSION['usuario']) || isset($_SESSION['senha'])){
        $usuariosql = $_SESSION['usuario'];
        $senhasql = $_SESSION['senha'];
        $dbcnx = @mysql_connect ('localhost', 'calesa', 'pass');
        mysql_select_db('calesa');
        $sqlprocura = "SELECT * FROM Admin WHERE Usuario = '$usuariosql' AND Senha = '$senhasql' ";
        $query = mysql_query($sqlprocura);
        $num = mysql_num_rows($query);
        if($num != "0"){
            header('location: index.php');
        }
        else{
            header('location: login.php');
        }
    }
    else{
        header('location: login.php');
    }
?>

then in the logoff script I set:

<?PHP
    session_start();
    unset($_SESSION['usuario']);
    unset($_SESSION['senha']);
    session_destroy();
    header('location: ../index.php');
?>

what is wrong cause it is not working and I´m getting to think I´m stupid cause I can´t understand php and I can´t make anything work.
Thanks for the help.

[devised]

Are you able to login? What exactly is happening when you try?

Also in the first block of code, try switching the variables that store the sessions around like this :

$usuario = $_SESSION['usuario'];
$senha = $_SESSION['senha']; 

And then change the form's action value like this:

<form name="form1" method="post" action="<?php $_SERVER['PHP_SELF']; ?>">

And on the block of code for each page of the admin, take out the quotes from around the zero like this:

        if($num != 0){