내 MySQLi 준비 명령문이 '비객체에서 멤버 함수 실행() 호출' 오류를 발생시키는 이유는 무엇입니까?

Fatal error: Call to a member function execute() on a non-object in ...

// Bind parameters to variables
$name = 'one';
$age = 1;
$stmt->bind_param('si', $name, $age);

<?php

// Establish database connection
$mysqli = new mysqli('localhost', 'root', 'root', 'test');

// Prepare statement
$stmt = $mysqli->prepare('INSERT INTO users (name, age) VALUES (?, ?)');

// Bind variables
$name = 'one';
$age = 2;
$stmt->bind_param('si', $name, $age);

// Insert data with prepared statement
$stmt->execute();

// Check for errors
if ($mysqli->error) {
    echo 'Error: ' . $mysqli->error;
} else {
    // Data inserted successfully
}

// Prepare select statement
$stmt = $mysqli->prepare('SELECT * FROM users WHERE name = ?');

// Bind variable
$name = 'one';
$stmt->bind_param('s', $name);

// Execute select query
$stmt->execute();

// Fetch results
$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
    echo $row['id'] . ' - ' . $row['name'] . ' - ' . $row['age'] . '<br>';
}

// Close statement and connection
$stmt->close();
$mysqli->close();

?>