wp_insert_post with category variable not working

hi .. i m encountering a strange problem in which i m inserting a post using the category variable .. here below is my working code ..

————————————
$new_post = array(
‘post_title’ => $title,
‘post_content’ => $description,
‘post_date’ => $post_date,
‘post_status’ => ‘future’,
‘post_category’ => array(18,19,20),
);

//SAVE THE POST
$pid = wp_insert_post($new_post);
————————————

this code works fine .. however when i try to add categories into an array with variable .. then it is only adding first category value into the post ..

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$new_array = array(18,19,20);
$final_cat_id = implode(‘,’,$new_cat_ID);
echo $final_cat_id;

$title = ‘title’;
$description = ‘description’;
$post_date = date(“Y-m-d H:i:s”);

// Create post object

$new_post = array(
‘post_title’ => $title,
‘post_content’ => $description,
‘post_date’ => $post_date,
‘post_status’ => ‘future’,
‘post_category’ => array($final_cat_id),
);

//SAVE THE POST
$pid = wp_insert_post($new_post);
————————————

i m getting 18,19,20 when i m echoing the $final_cat_id but it is only adding 18 category when inserting a post ..

can someone help me where i am going wrong . .