Questo è il file prenota.php

codice HTML:
<head>
    <title>Ospedale</title>id_
    <meta charset="utf-8">
    
    <script type = "text/javascript" src = "http://www.google.com/jsapi"></script>
    <script type = text/javascript> google.load('jquery', '2.2.0');
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js"  type="text/javascript"></script>
    <script type="text/javascript" src="js/slider.js"></script>
    
    <script type = text/javascript>
    
    $document.ready(function() {
        
        var scegli = '<option value="0">Effettua una scelta..</option>';
        var attendere = '<option value="0">Operazione in corso..</option>';
        
        $("select#esami").html(scegli);
        $("select#esami").attr("disabled", "disabled");
        $("select#prenota").html(scegli);
        $("select#prenota").attr("disabled", "disabled");
        
        $("select#reparti").change(function(){
            var reparto = $("select#reparti option:selected").attr('value');
                        
            $.post("php/select.php", {id_reparto:reparto}, function(data){
                $("select#esami").removeAttr("disabled"); 
                $("select#esami").html(data);    
            });
            });
            
        $("select#esami").change(function(){
            $("select#prenota").attr("disabled", "disabled");
            $("select#prenota").html(attendere);
            
            var esame = $("select#esami option:selected").attr('value');

            $.post("php/select.php", {id_esame:esami}, function(data){
                $("select#prenota").removeAttr("disabled");
                $("select#prenota").html(data);
                });
            });
            
        $("form#select_form").submit(function(){
            var reparto = $("select#reparti option:selected").attr('value');
            var esame = $("select#esami option:selected").attr('value');
            var data_p = $("select#prenota option:selected").attr('value');
        
        
            
    });
    </script>
    

    <link rel="stylesheet" type="text/css" href="css/stylesheet.css" />
    
    <style type="text/css">
    
     ul {
        list-style-type: none;
    
    }
        
    section fieldset {
    background-color: #1461A1;
    float: left;
    width: 640px;
    }
    
    label, p {
        color: white;
        
    }
        
    li {
        margin-bottom: 10px;
    }
    
    .asterisk {
        color: red;
    }
    
    label {
        margin-right: 30px;
        display: block;    
    }
    
    section #submit {
        height: 2em;
        width: 4em;
        margin-top: 20px;
    }
    
    </style>
    
    
</head>

<?php
    include_once ($_SERVER['DOCUMENT_ROOT']  . '/php/select.php');

?>

<body onload=start()>


    <header>
    
        <fieldset>
                <?php require($_SERVER['DOCUMENT_ROOT']. "/inc/users/account_opts.php"); ?>
        </fieldset>    
    
        <div class=logo>
                <img src="img/logo.jpg" alt="Ospedale" width="100" height="100" /> 
            <div id=textbox>
                <h1><span class="bigletter">O</span>spedale della<span class="bigletter"> m</span>urgia</h1>
                <h3><span>“F. Perinei” </span></h3>
            </div>
        </div>
        
        
    </header>
    
    
    
    <nav class=menu>
        <ul>
            <li class="default" onclick="selection(this);"><a href="index.php">Home</a></li>
            <li class="default" onclick="selection(this);"><a href="chisiamo.php">La struttura</a></li>
            <li class="default" onclick="selection(this);"><a href="reparti.php">I nostri reparti</a></li>
            <li class="default" onclick="selection(this);"><a href="prenotazioni.php">Prenotazioni</a></li>
            <li class="default" onclick="selection(this)"><a href="contatti.php">Info e Contatti</a></li>
        </ul>
    </nav>
    
    
    <div id=container>
        <div id=content>
            
            <section>                
                    <h3>Prenota i tuoi esami!</h3>
                    E' possibile prenotare i tuoi esami on line. Bastano pochi click!

                <p></p>
                
            <fieldset>
                
            <h2><label>Dati personali</label></h2>
            <form action="php/prenota.php" method="post" id="select_form" >
                <ul>
                    <li>
                        <label>Nome:</label>
                    <input name="nome" placeholder="nome" type="text" />
                    <span class="asterisk">*</span>
                    </li>
                    <li>
                        <label>Cognome:</label>
                        <input name="cognome" placeholder="cognome" type="text" />
                        <span class="asterisk">*</span>
                    </li>
                    <li> 
                        <label>Codice fiscale:</label>
                        <input name="cod_fiscale" placeholder="codice fiscale" type="text" />
                        <span class="asterisk">*</span>
                    </li>
                    <li> 
                        <label>Città:</label>
                        <input name="citta" placeholder="città" type="text" />
                        <span class="asterisk">*</span>
                    </li>
                
                    <p>
                        <label>Sesso</label>
                        <input type="radio" name="sesso" value="1"/>F<br/>
                        <input type="radio" name="sesso" value="2"/>M
                    </p>
                </ul>
         
                <h2><label>Dati impegnativa</label></h2>
                <ul>
                    <li>            
                        <label>Impegnativa:</label>
                        <input name="cod_impegnativa" placeholder="codice impegnativa" type="text" />
                        <span class="asterisk">*</span>
                    </li>
                </ul>
                
                 <h2>
                   <label>Reparti ----> Esami ----> Date disponibili</label><br/>
                 </h2>              
                  
                  <label> Seleziona reparto</label><br/>
                  
                    <select id="reparti">
                    <?php 
                        echo $opt->MostraReparti(); 
                    ?>
                    </select>


                    <label> Seleziona tipo esame</label><br/>
                    <select id="esami">
                        <option>Scegli..</option>
                    </select>
                   
                    <label> Seleziona il giorno tra quelli disponibili</label><br/>
                        <select id="prenota">
                            <option>Scegli..</option>
                        </select> 

                
                <p><input type="submit" name="submit" value="Prenota"></p>
                </form>
            
            </fieldset>
                
            </section>
        </div>
    </div>
   
</body>

Spero di aver rispettato le regole del forum e di essermi espressa in modo chiaro su quello che è il mio problema.
Grazie.