PHP + MySql +存储过程,如何获取"out"值？

DELIMITER //
CREATE PROCEDURE get_out_value(OUT out_param INT)
BEGIN
  SET out_param = 123;
END //
DELIMITER ;

<?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);
}

// 调用存储过程
$out_param = -1;
$sql = "CALL get_out_value(@out_param)";
$result = $conn->query($sql);

// 获取 "out" 参数
$sql = "SELECT @out_param";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
  while($row = $result->fetch_assoc()) {
    $out_param = $row["@out_param"];
    echo "out 参数值: " . $out_param;
  }
} else {
  echo "未找到 out 参数";
}

$conn->close();
?>