$data = array("number" => $_POST['policynum'], "mail" => $_POST['mail'], "phone" => $_POST['phone'] );
$data_string = json_encode($data);

$data = if(mail!="")
array("number" => $_POST['policynum'], "mail" => $_POST['mail']);
if(phone!="")
array("number" => $_POST['policynum'], , "phone" => $_POST['phone']);