mysqli_query() 期望参数 1 为 mysqli，给定 null？令人困惑？

Question

  define('DB_HOST', 'localhost');
  define('DB_USER', '******');
  define('DB_PASSWORD', '************');
  define('DB_NAME', '***********');

 $dbc = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);

 class User {

 function checkUser($uid, $oauth_provider,$username,$email,$twitter_otoken,$twitter_otoken_secret) 
{
    $query = mysqli_query($dbc,"SELECT * FROM si_table WHERE oauth_uid = '$uid' and oauth_provider = '$oauth_provider'");
    $result = mysqli_fetch_array($query);
    if (!empty($result)) {
        # User is already present
    } else {
        #user not present. Insert a new Record
        $query = mysqli_query($dbc,"INSERT INTO si_table (oauth_provider, oauth_uid, user_name,email_id,twitter_oauth_token,twitter_oauth_token_secret) VALUES ('$oauth_provider', $uid, '$username','$email')") ;
        $query = mysqli_query($dbc,"SELECT * FROM si_table WHERE oauth_uid = '$uid' and oauth_provider = '$oauth_provider'");
        $result = mysqli_fetch_array($query);
        return $result;
    }
    return $result;
}



}

我收到这个错误！我不知道为什么？

mysqli_query() 期望参数 1 为 mysqli，第 14、15、20、21 和 22 行中给出的为 null！

Answer 1

将 global $dbc; 添加到 checkUser 的开头。该变量未在函数作用域中定义。