Question

In a plugin, via the add_action() routine, I try to run a check. But getting the post ID as below doesn’t work, but why? What’s the correct way of getting the post ID and a related cat id?

add_action( 'wp', 'check_url', 10, 1 );

function check_url($wp){
    if( is_single() ){
        $cat_id = wp_get_post_categories( $post->ID );
    }
}

Answer 1

Add global to your function first before using $post to make it visible inside your function (this is why we love PHP):

function check_url($wp){
  global $post;
  ...
}