[wp_insert_post categories array, php array issue]

This code works:

 

$post_id = wp_insert_post( array(

'post_author' => $user_id,

                'post_category' => array(3) )

 

 

This does not

 

$post_id = wp_insert_post( array(

'post_author' => $user_id,

                'post_category' => array($category_id) )

 

Using echo, $category_id is indeed 3, so I don't get why this is not working. I'm guessing this has something to do with php and array creation but I couldn't figure out what is wrong.

What is the difference between array(3), and array($category_id) if $category_id is 3?