Con quella porzione di codice mi esce pagina bianca...
invece ho provato con questo e funziona correttamente.
[PHP]
<!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>
<link href="/archivio/stile.css" type="text/css" rel="stylesheet" />
<?php
$con=mysqli_connect("localhost","","","");
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
if(isset($_POST['Submit1'])){
$date_str = $_POST['id'];


$converted = date("Y-m-d", strtotime($date_str));


$totale_arresti=($_POST['categoria']);


$stranieri=($_POST['cognome']);


$flagranza=($_POST['data_nascita']);


$esecuzione=($_POST['motivo']);


$veicoli=($_POST['protocollo']);


$pg=($_POST['assegnazione']);


$result = mysqli_query($con, "UPDATE registro set categoria = '$totale_arresti', data_nascita='$flagranza', cognome='$stranieri', motivo='$esecuzione', protocollo='$veicoli',
assegnazione='$pg' where id=".$_GET["id"]);


$result1 = mysqli_query($con, "INSERT INTO assegna (id, assegnazione) SELECT id, assegnazione FROM registro where id = '" . $_GET["id"] . "'
");


if(!$result){
$message = "Problem in Editing! Please Retry!";
} else {
header("Location: check.php");
}
}


$result =mysqli_query ($con, "SELECT * FROM registro WHERE id = '" . $_GET["id"] . "'");
while ($riga = mysqli_fetch_array($result)){


?>
<script src="https://code.jquery.com/jquery-1.12.4.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>


<meta content="it" http-equiv="Content-Language" />
<meta content="text/html; charset=utf-8" http-equiv="Content-Type" />
<title>Scarico fascicolo</title>
</head>


<body style="width: 672px; height: 289px">


<h1 align ="center"><p>Scarico fasicolo</p></h1>
<div class="form">
<form method="post" action="<?php echo $PHP_SELF; ?>" style="width: 674px; height: 130px">
<fieldset>
<!-- Data presentazione:<input id= "data" value="<?php echo $riga['data']; ?> " name="data" type="date" /><br />-->
<label><b> Data:</label></b><input readonly="readonly" id ="data" name="data" value="<?php echo $riga['data']; ?> " type="text"/> <br />
<label><b> Categoria:</label></b><input readonly="readonly" id ="categoria" name="categoria" value="<?php echo $riga['categoria']; ?> "type="text" /> <br />
<label><b> Cognome:</label></b><input readonly="readonly" id ="cognome" name="cognome" value="<?php echo $riga['cognome']; ?> "type="text"onChange="javascript:this.value=this.v alue.toUpperCase();"/> <br />
<label><b> Data di Nascita:</label></b><input readonly="readonly" id ="data_nascita" name="data_nascita" value="<?php echo $riga['data_nascita']; ?> "type="text" onChange="javascript:this.value=this.value.toUpper Case();"/> <br />
<label><b> Motivo:</label></b> <input readonly="readonly"id ="motivo" name="motivo" value="<?php echo $riga['motivo']; ?> " type="text"onChange="javascript:this.value=this.va lue.toUpperCase();" /> <br />
<label><b> Protocollo:</label></b> <input readonly="readonly" id ="protocollo" name="protocollo" value="<?php echo $riga['protocollo']; ?> " type="text"onChange="javascript:this.value=this.va lue.toUpperCase();" /> <br />
<label><b> Assegnazione / Scarico: </label><input id ="assegnazione" name="assegnazione" value="<?php echo $riga['assegnazione']; ?> " type="text"onChange="javascript:this.value=this.va lue.toUpperCase();" /><br />
<button type="submit" name="Submit1">Invia</button>
</fieldset> </form></div>
<?php
}
?>
</body>
[\PHP]