Prova così....

<?php session_start();?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="it-it" lang="it-it" dir="ltr" id="minwidth" >
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>test</title>
<html>
<head><title></title></head>
<body>
<?php if(isset($_POST['invia'])){

$testo =$_POST['testo'];
$email = $_POST['email'];
$_SESSION['email'] = $email;

echo "Dati inviati al server:
";
echo "Sessione email :" .$_SESSION['email']."
";
echo "POST email :" .$email."
";
echo "POST test :" .$testo."
";

echo "Ora puoi effettuare validazioni";

//qui le validazioni dei campi del form....
}else{


<form method="post" action="<?php $_SERVER['PHP_SELF'];?>">
<span>Email:</span>
<input name="email" type="text" title="Inserisci il tuo indirizzo email" size="50" maxlenght="50" />


<span>Oggetto:</span>
<select name="oggetto">
<option value="vuoto" selected="selected"></option>
<option value="opz_1">Opzione 1</option>
<option value="opz_2">Opzione 2</option>
</select>


<span>Testo:</span>
<textarea name="testo" title="Scrivi il tuo testo" rows="13" cols="61"></textarea>


<input type="submit" name="invia" value="Invia email" />

</body>
</html>
<?php }?>