Modifica il codice di form.php in questa maniera
Codice PHP:

<?php     
if (isset($_POST['element_1_1']) && isset($_POST['element_1_2']) && isset($_POST['element_1_3']) && isset($_POST['element_3']) && isset($_POST['element_4_1']) && isset($_POST['element_5']) && isset($_POST['element_7_1']) && isset($_POST['element_7_2']) && isset ($_POST['element_8']) && isset($_POST['element_8']))   
{
              
$nome strip_tags(addslashes($_POST['element_4_1']." ".$_POST['element_4_2']));     
$date = (int) $_POST['element_1_1'] . "/".(int) $_POST['element_1_2'] . "/".(int) $_POST['element_1_3'];        
 
$Time strip_tags(addslashes($_POST['element_7_1']." ".$_POST['element_7_2'] ));         $email strip_tags(addslashes($_POST['element_3']));      
$club strip_tags(addslashes($_POST['element_5']));      
$giocatori strip_tags(addslashes($_POST['element_8']));         
$telefono strip_tags(addslashes($_POST['element_9']));        
 
$to "indirizzoemail";        
 
$subject "Prenotazione";                 
 
$body "             Nome E Cognome : ".$nome." \n             arrivo : ".$date." \n             Ora : ".$Time." \n             Email : ".$email." \n             squadra : ".$club." \n             Quanti giocatori : ".$giocatori." \n             Telefono : ".$telefono." \n         ";          $headers "From: $to \n            ";                          
if(
mail($to$subject$body$headers)) {  //modificate il messaggio a vostro piacere 
echo ("Mail inviata con successo");  } 
 
//nel caso ci fossero problemi 
 
else 
 { echo (
"Si è verificato un errore durante l'invio."); }      } ?>    
<!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"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Untitled Form</title> <link rel="stylesheet" type="text/css" href="view.css" media="all"> <script type="text/javascript" src="view.js"></script> <script type="text/javascript" src="calendar.js"></script> </head> <body id="main_body" >          [img]top.png[/img]     <div id="form_container">              <h1><a>Untitled Form</a></h1>         <form id="form_399422" class="appnitro"  method="post" action="">                     <div class="form_description">             <h2>Untitled Form</h2>             

This is your form description. Click here to edit.</p>         </div>                                     <ul >                                  <li id="li_4" >         <label class="description" for="element_4">Nome e Cognome  </label>         <span>             <input id="element_4_1" name= "element_4_1" class="element text" maxlength="255" size="8" value=""/>             <label>First</label>         </span>         <span>             <input id="element_4_2" name= "element_4_2" class="element text" maxlength="255" size="14" value=""/>             <label>Last</label>         </span><p class="guidelines" id="guide_4">[size="1"]Please insert your name and surname /per favore scrivi il tuo nome che vale per la prenotazine[/size]</p>                     <li id="li_1" >         <label class="description" for="element_1">Date </label>         <span>             <input id="element_1_1" name="element_1_1" class="element text" size="2" maxlength="2" value="" type="text"> /             <label for="element_1_1">MM</label>         </span>         <span>             <input id="element_1_2" name="element_1_2" class="element text" size="2" maxlength="2" value="" type="text"> /             <label for="element_1_2">DD</label>         </span>         <span>              <input id="element_1_3" name="element_1_3" class="element text" size="4" maxlength="4" value="" type="text">             <label for="element_1_3">YYYY</label>         </span>              <span id="calendar_1">             [img]calendar.gif[/img]             </span>         <script type="text/javascript">             Calendar.setup({             inputField     : "element_1_3",             baseField    : "element_1",             displayArea  : "calendar_1",             button         : "cal_img_1",             ifFormat     : "%B %e, %Y",             onSelect     : selectDate             });         </script>                           <li id="li_7" >         <label class="description" for="element_7">Time </label>         <span>             <input id="element_7_1" name="element_7_1" class="element text " size="2" type="text" maxlength="2" value=""/> :              <label>HH</label>         </span>         <span>             <input id="element_7_2" name="element_7_2" class="element text " size="2" type="text" maxlength="2" value=""/> :              <label>MM</label>         </span>                                  <li id="li_3" >         <label class="description" for="element_3">Email </label>         <div>             <input id="element_3" name="element_3" class="element text medium" type="text" maxlength="255" value=""/>          </div><p class="guidelines" id="guide_3">[size="1"]Please fill form using valid email address/inserisci un valido indirizzo, servirà per contattarti per eventuali problemi[/size]</p>                  <li id="li_5" >         <label class="description" for="element_5">Golf Club </label>         <div>             <input id="element_5" name="element_5" class="element text medium" type="text" maxlength="255" value=""/>          </div>                  <li id="li_8" >          <label class="description" for="element_8">Quanti persone </label>         <span>             <input id="element_8_1" name="element_8" class="element radio" type="radio" value="1" /> <label class="choice" for="element_8_1">1</label> <input id="element_8_2" name="element_8" class="element radio" type="radio" value="2" checked="checked"/> <label class="choice" for="element_8_2">2</label> <input id="element_8_3" name="element_8" class="element radio" type="radio" value="3" /> <label class="choice" for="element_8_3">3</label> <input id="element_8_4" name="element_8" class="element radio" type="radio" value="4" /> <label class="choice" for="element_8_4">4</label>          </span>                  <li id="li_9" >         <label class="description" for="element_9">Telefono </label>         <div>             <input id="element_9" name="element_9" class="element text medium" type="text" maxlength="255" value=""/>          </div>                                            <li class="buttons">                 <input type="hidden" name="form_id" value="399422" />                                  <input id="saveForm" class="button_text" type="submit" name="submit" value="Submit" />[/list]</form>             <div id="footer">             Generated by [url="http://www.phpform.org"]pForm[/url]         </div>     </div>     [img]bottom.png[/img]     </body> </html>
Ricorda, inoltre, di inserire un isset nel 1° if per ogni campo che usi ( controlla i name di tutti i campi, e i radio li devi prendere una volta sola. La prima regola da osservare, seppure dura, è non fidarsi mai degli utenti