echo '<div id="dialog-User" title="Для подписи:">
 <form id="users_podp">
<fieldset>
<input name="id_dug" id="id_dug" type="hidden" value="">';
		if ($users = $link->query('SELECT id_user, name FROM login')) { 
    while( $row = $users->fetch_assoc() ){ 
		echo '<input type="checkbox" name="otvetstv['.$row['id_user'].']" id="otvetstv['.$row['id_user'].']" value="'.$row['id_user'].'">
		<label for="otvetstv['.$row['id_user'].']">'. $row['name'].'</label><br>';
    } 

function saveform(){	
	var data = $('#my').serialize();
	$.post("save.php", data, function(data) {       
		document.getElementById("id_dug").value=data;
   		alert ("Счет сохранен"); 
	});	
}

.........
if ( bValid ) {
		var data = $('#users_podp').serialize();
			$.post("save_user_podp.php", data, function(data) {        
				alert ("данные сохранены  "); 
			});
.......

document.getElementById("id_dug").value=123; 

..................
$info = $link->prepare("INSERT INTO dugov_info VALUES (null,?,?,?,?,?,?,?,?,?,?,?)");
$dt =  date("Y.m.d");

	$info->bind_param('sisdddddddi', $_POST['Name_pechi'], $_POST['Name_zakaz'], $dt , $_POST['rub'], $_POST['dollar'], $_POST['euro'], $_POST['tamogn'], $_POST['transport'],  $_POST['sklad'], $_POST['strahovan'],$sign );
	$info->execute();
	$info->store_result();
	$id_S = $info->insert_id;
	$info->close();
.............................
echo ($id_S);