$insert = \"INSERT INTO event_tracker_table (event_name, event_location, location_number, event_creator_username, event_creator_email)
VALUES (
Insert id is a property of the MYSQLI object and not the MYSQLI result object:
$statement = $mysqli->query($insert);
echo $mysqli->insert_id; // correct
echo $statement->insert_id; //not correct
http://php.net/manual/en/mysqli.insert-id.php
You can get the auto_increment value with
$id = mysqli_insert_id($mysqli);
See mysqli_insert_id for more info.