我如何计算mysql查询返回的行数？

SELECT COUNT(*) FROM users WHERE age > 18;

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接
if ($conn->connect_error) {
  die("连接失败: " . $conn->connect_error);
}

// 执行查询并获取结果
$sql = "SELECT COUNT(*) FROM users WHERE age > 18";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
  // 输出结果
  while($row = $result->fetch_assoc()) {
    echo "年龄大于 18 岁的用户数量: " . $row["COUNT(*)"];
  }
} else {
  echo "0 结果";
}

$conn->close();
?>