wp_set_post_categories not working for me as should

Hello,

I’ve tried to run a PHP file created by me.

It has one purpose only: to set some categories to an existing post.

I use (wrongly, it seems) this function: wp_set_post_categories

Cand you plese guide me into one solution?

See bellow my (not working) PHP file. I’ve put it in the Plugins folder, to allow me to run admin.php.

<?php

$cats = array ("cat1","cat3","cat2");  //these categories exist

$admin = realpath(dirname(__FILE__) . '/../..'.'/wp-admin');   //this line works, no erros given
chdir($admin);                                                    //this line works, no erros given
require_once $admin . '/admin.php';                               //this line works, no erros given

$id = intval(63);  //I use this function to make sure it's an integer value

wp_set_post_categories($id, $cats);  //you got it, this doesn't work

echo 'I got until here.';  //This text is printed on the screen

?>

Any ideas welcome.