Notice: Undefined index [duplicate]

Answer 1

isset() returns a boolean value: TRUE / FALSE. If you want to determine whether $_GET["id"] is set, you need to do so in an IF-Statement, or via a ternary operator.

if (isset($_GET["id"])) {
  $thread_id = $_GET["id"]; // Bad idea taking values in from $_GET directly
} else {
  die("Thread id not set");
}