PDO LastInsertId returning nothing - pdo

Question

For some reason I cannot get the LastInsertId from my PDO insert. I am receiving nothing back, if I place it inside the execute I get -1 because the insert query hasn't run. However when just after the execute to grab the last ID inserted nothing is returned.
php 5.1.6
PECL pdo = 0.1.0
I have looked at following questions and many other stack exchange questions.
lastInsertId does not work in Postgresql
PDO lastInsertId() returns 0
PDO lastInsertId issues, php
However the difference is nothing is returned compared to 0. No errors are recorded either. See the code below.
Connection
try {
$conn = new PDO("pgsql:host=localhost port= dbname=", "", "");
echo "PDO connection object created";
}
catch(PDOException $e) {
echo $e->getMessage();
}
insert / return the last id
$stmt = $conn ->prepare("INSERT INTO sheet_tbl (site_id, username, additionalvolunteers) VALUES(?,?,?)");
$stmt->bindParam(1,$site_id);
$stmt->bindParam(2,$username1);
$stmt->bindParam(3,$additionalvolunteers);
$site_id = $_POST['site_id'];
$username1 = $user->name;
$additionalvolunteers = $_POST['additionalvolunteers'];
$stmt ->execute();
$newsheetID = $conn->lastInsertId('sheet_id');
echo $newsheetID . "last id";

Answer 1

You should be using
$newsheetID = $conn->lastInsertId('sheet_id_**seq**');
Just add _seq after id.

Answer 2

Assumming sheet_id type is SERIAL you can use LASTVAL()
TRY (Not tested as I dont have postgresql)
$newsheetID = $conn->query("SELECT LASTVAL()");
echo $newsheetID . "last id";

Answer 3

I use postgresql in C# with the "RETURNING rowkey" and its has always worked for me.
I also use pdo/php in a lot of applications.
However i dont have a postgres database open to the server right now for testing.
This could should work but it is not tested.
$site_id = $_POST['site_id'];
$username1 = $user->name;
$additionalvolunteers = $_POST['additionalvolunteers'];
$stmt = $conn ->prepare("INSERT INTO sheet_tbl (site_id, username, additionalvolunteers) VALUES(:site_id, :username1, :additionalvolunteers) RETURNING row_id");
$success = $stmt->execute([
'site_id' => $site_id,
'username1' => $username1,
'additionalvolunteers' => $additionalvolunteers
]);
// unsure about this
$newsheetID = $stmt->fetchColumn();
echo $newsheetID . " last id";