<html>

<head>
<title> Form </title>
<style type="text/css">
/* id bambini */

#contenitore{
width:700px;
height:30px;
}

#contbamb{
width:500px;
}

#sel1{
width: 50px;
/*padding-right: 20px;*/
text-align: left;
height: 25px;
float: left;
}

#sel2{
width: 180px;
padding-left: 20px;
text-align:left;
height: 25px;
float: left;
}

</style>


<!-- script che in base al numero dei bambini visualizza eta bambini -->
<script language="javascript" type="text/javascript" src="jquery-1.11.3.js"></script>

<script type="text/javascript">
$(document).ready(function(e) {
//funzione che cattura valore prima select e richiamo della funzione che permette di stampare select eta
$("#bambini").on('change', function(){
var qt = $(this).val();
SelectEta(qt);
})
});

function SelectEta(val){
$("#sel2").html('');
$("#sel2").append ("<table cellpadding='10' cellspacing='0'><tr></tr></table>")
for(var i=0; i<val; i++){
$("#sel2").append('<td><select name="etabambini[]" id="etabambini"'+i+'"><option value ="0"> 0 </option><option value ="1"> 1 </option><option value ="2"> 2 </option><option value ="3"> 3 </option><option value ="4"> 4 </option><option value ="5"> 5 </option><option value ="6"> 6 </option><option value ="7"> 7 </option><option value ="8"> 8 </option><option value ="9"> 9 </option></select></td>&nbsp;&nbsp;');
}
}


</script>

</head>

<body>
<!-- Creazione form -->
<h2>Calcola Preventivo</h2>
<form name="myform" action="calcola.php" method="post">
<!--Nome: <input type = "text" name = "nome"> &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&n bsp;&nbsp;&nbsp;
Cognome <input type="text" name = "cognome"><br><br><br>
Citta <input type="text" name = "citta">&nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&n bsp;&nbsp;&nbsp;
Telefono <input type = "text" name = "telefono">&nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&n bsp;&nbsp;
Email <input type= "text" name="email"><br><br><br>-->
<div id="contenitore">
Check-in <input type="date" name="data_checkin">&nbsp;
Check-out <input type="date" name="data_checkout"> &nbsp;

<!-- Camere -->
Camere
<select id="camere" name="camere">
<option value ="1"> 1 </option>
<option value ="2"> 2 </option>
<option value ="3"> 3 </option>
<option value ="4"> 4 </option>
<option value ="5"> 5 </option>
</select>

<!-- Adulti -->
Adulti
<select id="adulti" name="adulti">
<option value ="1"> 1 </option>
<option value ="2"> 2 </option>
<option value ="3"> 3 </option>
<option value ="4"> 4 </option>
<option value ="5"> 5 </option>
</select>

<br><br> Bambini
<div id="contbamb">
<div id="sel1">
<select id="bambini" name="bambini">
<option value ="0"> 0 </option>
<option value ="1"> 1 </option>
<option value ="2"> 2 </option>
<option value ="3"> 3 </option>
<option value ="4"> 4 </option>
<option value ="5"> 5 </option>
</select>
</div>
<div id="sel2"><select name="etabambini"><option value="0">0</option><option value="1">1</option>
<option value ="2"> 2 </option><option value ="3"> 3 </option><option value ="4"> 4 </option><option value ="5"> 5 </option>
<option value ="6"> 6 </option><option value ="7"> 7 </option><option value ="8"> 8 </option><option value ="9"> 9 </option>
</select></div>
</div>
<!--Trattamento-->
<br><br>
<div style="float: left;">
<b>Trattamento </b>
<select id="trattamento" name="trattamento">
<option value ="pensionecompleta"> Pensione Completa </option>
<option value ="mezzapensione"> Mezza Pensione </option>
<option value ="beb"> Bed & Breakfast </option>
</select> &nbsp;

<!-- Servizio spiaggia -->
<b> Servizio spiaggia</b>
<select id="spiaggia" name="spiaggia">
<option value ="niente"> - </option>
<option value ="standard"> Standard </option>
<option value ="primafila"> Prima Fila </option>
</select> &nbsp;

<!-- Extra-->
<b> Supplemento culla </b>
<input type="checkbox" id="culla" name="culla" value="supplemento culla"/>
<br><br>
<input type ="submit" value="Calcola Preventivo" style="background-color:#ABCDEF;">
</div>
</div>
</form>

</body>
</html>
PHP (PRIME RIGHE DI CODICE)

<?php


session_start();

// variabili
$costotratt=0;$sommabambad=0;

//assegnazione variabili utente tramite metodo POST
//servizi extra

$checkin = trim ($_POST ["data_checkin"]);
$checkout = trim ($_POST ["data_checkout"]);
$camere = trim($_POST["camere"]);
$adulti = trim ($_POST ["adulti"]);
$bambini = trim ($_POST ["bambini"]);
$trattamento = trim ($_POST ["trattamento"]);
$sommabambad = $adulti + $bambini;
$servspiaggia = trim ($_POST["spiaggia"]);
?>