Hai aperto i tag php in maniera inutile
Correggi così:
Codice PHP:
<?php 
 $stat_pag 
$pag

      switch (
$stat_pag) { 
                        case 
'Acconto'
                        echo 
'Ha dato un acconto'
                        echo  
                                    
'<form action="sql_invpag.php" method="get"> 
                                        <input name="aggiorna" type="submit" value="Pagato" /> 
                                        <input type="hidden" value="'
.$_GET["id"].'"  name="id" /> 
                                    </form>  '


                              
                        break; 

                        case 
'Pagato'
                        echo 
'Ha Pagato Tutto.'
                        break; 

                        default: 
                        echo 
'Deve Pagare!'
                        break; 
                    } 
                
?>