Passing a PHP variable into Javascript returns NULL

I know there are a lot of questions covering something similar, but I've tried the answers without any luck.

Snippet of PHP:

$usernum_query = "SELECT numPersonID FROM tbllogins WHERE txtUserName='$currentuser'";

if(!$usernumber = $db1->query($usernum_query)){
    die('There was an error running the usernumber query [' . $db1->error . ']');
}

while($row = $usernumber -> fetch_assoc()) {
  $userIDnum = $row['numPersonID'];
  $userIDnum = utf8_encode($userIDnum);
}

Snippet of Javascript:

$(function(){

  $("#0").click(function(){

        var userIDnum = <?php echo json_encode($userIDnum); ?>;
        alert(userIDnum);

The most common answer I've come across says to UTF_encode my variable, which I think I've done correctly. I've also tried:

var userIDnum = <?php echo $userIDnum; ?>;

Which doesn't work.

In my HTML outside of the script,

<?php echo json_encode($userIDnum); ?>

return "90" (with the quotes)

<?php echo $userIDnum; ?>

returns 90 (without the quotes).

Within the script, I get null and no alert box, respectively.

Any ideas as to why the variable isn't passed into the script? Thanks!

edit: tried with quotes and got the same result