如何修复未定义的变量通知

Question

我有一个调用函数的php文件，并且从函数中收到“未定义的变量”通知。

PHP：

$secret_key = random_password_generate(32);
$secret_iv = random_password_generate(16);
$encrypted_name = encrypt_decrypt('encrypt', $name); 

PHP函数：

function encrypt_decrypt($action, $string)
    {
    $output = false;
    $encrypt_method = "AES-256-CBC";
    $key = hash('sha256', $secret_key);

    $iv = substr(hash('sha256', $secret_iv), 0, 16);
    if ($action == 'encrypt') {
        $output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
        $output = base64_encode($output);
    } else if ($action == 'decrypt') {
        $output = openssl_decrypt(base64_decode($string), 
    $encrypt_method, $key, 0, $iv);
    }
    return $output;
    }

我收到的通知是：

Notice: Undefined variable: secret_key in /myPathTo/functions.php on line 75

Notice: Undefined variable: secret_iv in /myPathTo/functions.php on line 77

有什么建议吗？

谢谢。