PHP遵循系统查询错误

Question

我目前正在实施类似于twitter的关注系统。问题是其中一个查询似乎不起作用。

我有两个数据库; tbl_users并关注。

tbl_users包含：userId，userName，userEmail，userPass和followers_count

关注包含：follow_id，接收者，发件人和状态

在USER类中，我有以下功能：

 //get all users from database where userID not = your id
 public function users($user_id){
 global $pdo;
 $query = $pdo->prepare("SELECT * FROM `tbl_users` U LEFT JOIN `follow` F on 
 `f`.`receiver` = `U`.`userID` AND CASE WHEN `F`.`sender` = ? THEN 
 `F`.`receiver` = `U`.`userID` END where `U`.`userID` != ?");
 $query->bindValue(1,$user_id);
 $query->bindValue(2,$user_id);
 $query->execute();
 return $query->fetchAll(PDO::FETCH_ASSOC);
 }

在我的index.php中，我调用以下代码：

$user_home = new USER();
$user_id = $_SESSION['userID'];
$users = $user_home->users($user_id);

返回此错误：

 [29-Aug-2017 05:17:05 UTC] PHP Fatal error:  Uncaught Error: Call to a
 member function prepare() on null in
 /home/iamlive/public_html/class.channel.php:143 
Stack trace:
 #0 /home/iamlive/public_html/index.php(25): USER->users('1')
 #1 {main} thrown in /home/iamlive/public_html/class.channel.php on line 143