确定PHP mysqli fetch是否返回数据

I'm trying to take the following code from the PHP website and rewrite to echo out if the returns my SQL data is null.

<?php
 $mysqli = new mysqli("localhost", "my_user", "my_password", "world");

/* check connection */
if (mysqli_connect_errno()) {
   printf("Connect failed: %s
", mysqli_connect_error());
   exit();
}

 $city = "Amersfoort";

 /* create a prepared statement */
 if ($stmt = $mysqli->prepare("SELECT District FROM City WHERE Name=?")) {

/* bind parameters for markers */
$stmt->bind_param("s", $city);

/* execute query */
$stmt->execute();

/* bind result variables */
$stmt->bind_result($district);

/* fetch value */
$stmt->fetch();

printf("%s is in district %s
", $city, $district);

/* close statement */
$stmt->close();
}

/* close connection */
$mysqli->close();
?>

I'm having issues figuring out the right conditional to do this. Just looking for idea's on how to further optimize this code.

$rows = $stmt->rowCount();
echo $rows;

If you are trying to find out whether $district variable is null after the execution of the SQL query, you can try:

if(!empty($district)){

      //code if there is a value

 } 
 else{

     //code if it is null
  }

If you are sure that the database returns null if not found, you can use !is_null($district) instead of !empty($district).