mysqli プリペアド ステートメントが「非オブジェクトのメンバー関数execute() の呼び出し」を返すのはなぜですか?

<?php

// Establish database connection
$mysqli = new mysqli("localhost", "root", "root", "test");
if ($mysqli->connect_errno) {
    echo "Failed to connect to MySQL: " . $mysqli->connect_error;
    exit;
}

// Prepare insert statement
$stmt = $mysqli->prepare("INSERT INTO users (name, age) VALUES (?,?)");
if (!$stmt) {
    echo "Error preparing statement: " . $mysqli->error;
    exit;
}

// Bind parameters and insert one row
$name = 'One';
$age = 1;
$stmt->bind_param('si', $name, $age);
$stmt->execute();
if ($stmt->errno) {
    echo "Error executing insert: " . $stmt->error;
    exit;
}

// Insert another row with different values
$name = 'Two';
$age = 2;
$stmt->bind_param('si', $name, $age);
$stmt->execute();
if ($stmt->errno) {
    echo "Error executing insert: " . $stmt->error;
    exit;
}

// Prepare select statement
$stmt = $mysqli->prepare("SELECT name, age FROM users");
if (!$stmt) {
    echo "Error preparing statement: " . $mysqli->error;
    exit;
}

// Execute select statement and fetch results
$stmt->execute();
$result = $stmt->get_result();
if ($result->num_rows > 0) {
    while ($row = $result->fetch_assoc()) {
        echo "{$row['name']} is {$row['age']} years old<br>";
    }
}

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