How to update global variables in plugin activation callback?

Question

Sorry for being such a noob. I'm sure this is an easy question for most. I'm trying to register activation and deactivation hooks for my plugin. In the docs, they warn about the activation callback not having access to global variables:

---

If you’re using global variables, you may find that the function you pass to register_activation_hook() does not have access to global variables at the point when it is called, even though you state their global scope within the function like this:

...

The main body variables are therefore in the scope of the activate_plugin() function and are not global unless you explicitly declare their global scope:

global $myvar;
$myvar = 'whatever';

function myplugin_activate() {

   global $myvar;
   echo $myvar; // this will be 'whatever'
}
register_activation_hook( __FILE__, 'myplugin_activate' );

---

So, following that advice, I set up my code like this:

global $post_id;
$post_id = 0;

function install_xxx() {
    global $post_id;

    $post = array(
        'post_title'    => __( 'Thank You', 'xxx' ),
        'post_content'  => __( 'Lorem ipsum dolor.', 'xxx' ),
        'post_status'   => 'publish',
        'post_name'     => 'thank-you',
        'post_type'     => 'page'
    );

    $post_id = wp_insert_post( $post, true );
    error_log('Inserted page: ' . $post_id);
}

function uninstall_xxx() {
    global $post_id;
    error_log('Removing page: ' . $post_id);
    $post = wp_delete_post( $post_id, true );
}

register_activation_hook( __FILE__, 'install_xxx' );
register_deactivation_hook( __FILE__, 'uninstall_xxx' );

But in my logs, it will echo something like:

• Inserted page: 109
• Removing page: 0

And, of course, the page isn't removed.

In other words, the global $post_id isn't being updated. Any clue what I'm doing wrong?

Answer 1

Thanks to @Tom J Nowell's help, I fixed my code to persist the created page ID to Options:

function install_xxx() {    
    $post = array(
        'post_title'    => __( 'Thank You', 'xxx' ),
        'post_content'  => __( 'Lorem ipsum dolor.', 'xxx' ),
        'post_status'   => 'publish',
        'post_name'     => 'thank-you',
        'post_type'     => 'page'
    );

    $post_id = wp_insert_post( $post, true );
    add_option( 'xxx_thankyou_page_id', $post_id );
    error_log('Inserted page: ' . $post_id);
}

function uninstall_xxx() {
    $post_id = get_option( 'xxx_thankyou_page_id' );
    error_log('Removing page: ' . $post_id);
    if ( $page_id ) {
        $post = wp_delete_post( $post_id, true );
    }
}

register_activation_hook( __FILE__, 'install_xxx' );
register_deactivation_hook( __FILE__, 'uninstall_xxx' );