getting form inputs values and passing to php variables

Question 1

You have access to the submitted form values via the $_POST superglobal.

For example, if your form field is named amount, you can access the value using $_POST['amount']. That's the value you can assign to the $amount variable in your script.

You don't need the global $_POST; line.

Reference: http://php.net/manual/en/reserved.variables.post.php

EDIT: When you deal with form input, do not forget to sanitize it.

Question 2

I finally solved it. I had to move input visible fields (which the user has to populate) to another firstly form.

firstform.php

<form action="tpv.php" method="POST">
<input type='text' name='requiredinput1' id='requiredinput1' placeholder='Introduzca nºfactura y concepto' style='width: 250px;height: 30px;'/><br> 
<input type='text' name='requiredinput2' id='requiredinput2' placeholder='Introduzca el importe de la factura' style='width: 250px;height: 30px;margin-top: 1em;'/> <br>
<input type='text' name='requiredinput3' id='requiredinput3' placeholder='Introduzca email' style='width: 250px;height: 30px;margin-top: 1em;margin-bottom: 1em;'/> <br>
<input type="submit" value="Realizar pago" />
</form>

This is the first, in which I populate three data (description , amount, email) , and send them to tpv.php . So, in tpv.php , I pick up them by using $_POST

tpv.php

<?PHP


// If form is submitted with all required data then show the form
// else show error page
empty($Formulario) ?              
    ShowForm($Ds_Merchant_Amount,$Ds_Merchant_Currency,$prod) :
    ShowError();
exit;
?>

<?PHP

function ShowError () {
  echo "<table width=100% height=50%><tr><td><p><h2><center>Compruebe que todos los datos del formulario son correctos!!</center></h2></p></td></tr></table>\n";
} # End of function ShowError

function ShowForm ($amount,$currency,$producto) {
// Posted data
global $_POST;

// Valores constantes del comercio
$url_tpvv='xxxx';
$clave='xxxx';
$name='Panel piedra';
$code='xxxx';
$terminal='1';
$order=date('ymdHis');
$amount = $_POST['requiredinput2']; //importe
$currency='978';
$transactionType='0';
$urlMerchant=''; //ruta a fichero que notifica por email
$producto = $_POST['requiredinput1'];


// Now, print the HTML script
echo "

<body bgcolor=white>
<form name=compra action=$url_tpvv method=post target=tpv>
<pre>
<table>
<tr><td>";

echo "</td>
</tr><tr><td>


<input type='hidden' name='Ds_Merchant_Amount' value='$amount' />
</td></tr><tr><td>
<input type=hidden name=Ds_Merchant_Currency value='$currency'>
</td></tr><tr><td>
<input type=hidden name=Ds_Merchant_Producto value='$producto'>
</td></tr><tr><td>
<input type=hidden name=Ds_Merchant_Order  value='$order'>
</td></tr><tr><td>
<input type=hidden name=Ds_Merchant_MerchantCode value='$code'>
</td></tr><tr><td>
<input type=hidden name=Ds_Merchant_Terminal value='$terminal'>
</td></tr><tr><td>
<input type=hidden name=Ds_Merchant_TransactionType value='$transactionType'>
</td></tr><tr><td>
<input type=hidden name=Ds_Merchant_MerchantURL value='$urlMerchant'>
</td></tr><tr><td>";

// Compute hash to sign form data
// $signature=sha1_hex($amount,$order,$code,$currency,$clave);
$message = $amount.$order.$code.$currency.$transactionType.$urlMerchant.$clave;
$signature = strtoupper(sha1($message));

echo "<input type=hidden name=Ds_Merchant_MerchantSignature value='$signature'>
</td></tr>
</table>

</pre>
</form>



<script language=JavaScript>  
    vent=window.open('','tpv','width=725,height=600,scrollbars=no,resizable=yes,status=yes,menubar=no,location=no');
    document.forms[0].submit();

</script>

";
} # End of function ShowForm
?>

I hope that this answer helps others users. Regards.