post__in not recognizing multiple IDs

Question

This is more of a PHP than a WordPress issue:

If $newstring = '15,30'; then array($newstring) is an array containing this string.

If you print_r( array($newstring) ), it looks like this:

Array (
    [0] => 15,30
)

Which is not the same as array(15,30).

What you want to do instead is:

$newstring = '15,30';
$args = array(
    'post__in' => explode(',', $newstring)
);

print_r( explode(',', $newstring) ) will look like:

Array (
    [0] => 15
    [1] => 30
)

Which now is exactly the same as array(15,30).

See explode() to better understand how this function works.