How to fix Notice: Undefined index: uid in?

Question

PHP

Error when calling uinfo function Error itself: Notice : Undefined index: uid in .... Function itself:
<?php if(uinfo($_SESSION['uid'],"who") == 2) {?>

function uinfo($uid,$value) {
  global $con;
  $sql = $con->prepare("SELECT * FROM users WHERE username =?");
        $sql->execute(array($uid));
  $row = $sql->fetch(PDO::FETCH_ASSOC);
  return $row[$value];
}

The fact is that the error disappears during authorization. there is a session. But the error appears if the user is not authorized.
isset throws another error annot use isset() on the result of an expression (you can use "null !== expression" instead)
How can I fix this error?

Answer 1

$_SESSION['uid'] !== null

Answer 2

php.net/manual/ru/function.empty.php
php.net/manual/ru/function.isset.php
php.net/manual/ru/function.array-key-exists.php

Answer 3

for example, in the function check
if(!is_null($uid)) {
.. database query
}