为什么此代码不显示错误消息？

Question

如果有人尝试按下按钮时任何字段为空，我想显示一条错误消息，但是当我将字段保留为空时，什么也没有发生，表单只是添加到具有空字段的数据库中。

所以我想知道代码有什么问题，删除或更改了哪些内容。

    <?php 

     $servername ="localhost";
     $username = "root";
     $password = "";
     $db       = mysqli_connect("localhost", "root", "");
     $db->select_db('hgeza06');

     $mysqli  = mysqli_connect($servername, $username, $password);
/*
   $con = mysqli_connect("localhost","root","");
if (!$con)
  {
  die('Could not connect: ' . mysqli_error());
  } else {
      echo "Succes!";
  }

*/

mysqli_select_db( $mysqli ,"hgeza06");

  if (isset($_POST['submit']))  
{ 
$address   = mysqli_real_escape_string($mysqli, $_POST['address']);
$password  = md5(mysqli_real_escape_string($mysqli,$_POST['password']));
$password1 = md5(mysqli_real_escape_string($mysqli,$_POST['password1']));


$sql    = "INSERT INTO cegek(Email, Password)VALUES('$address', '$password')";

$result = mysqli_query( $mysqli,$sql) or trigger_error(mysqli_error($mysqli)); 


if(empty($_POST['address'])){
    echo "Email is required!";
}

if(empty($_POST['password'])){
    echo "Password is required!";
}

if(empty($_POST['password1'])){
    echo "Password is required!";
}


header("location: http://localhost/phptest/reg/reg.php");
exit;
}


?>