致命错误：在第16行上的非对象上调用成员函数prepare（）

我的网站上有注册系统，并且我将所有的mysql语句都更改为PDO语句。更改它时，出现以下错误：致命错误：在第16行的非对象上调用成员函数prepare（）

这是我的代码...

<?php

include("sql.php");
    require("includes/password.php");

session_start(); //Start session for writing

$errmsg = array(); //Array to store errors

    $noterr = array();

$errflag = false; //Error flag

function UniqueID() {
$UID = rand(); //Create unique ID
$check = $db->prepare('SELECT * FROM `users` WHERE `UID` = :UID'); //line 16

    $UIDarray = array(
        UID => $UID
);

    $check->execute($UIDarray);

sql.php ...

<?php

ob_start();
session_start();

//database credentials
$dbhost = 'dbhost';
$dbuser = 'dbuser';
$dbpass = 'dbpass';
$dbname = 'dbname';

$db = new PDO("mysql:host=$dbhost;dbname=$dbname", $dbuser, $dbpass);
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
?>