Add a filter inside an action init

Question

Hello in the init action, I would like to add a filter to a fairly large function to modify variables in the $vars array, for example the Wordpress post ID.

That is to say:

    add_action( 'init',function(){
        //code
    
        add_filter( 'query_vars',function($vars){
            $vars[] = array('ID' => $myid);
             return $vars;
    
        });
    });

Is this possible?

EDIT: I am doing A/B/C tests of pages and with the same url I want to show a page with another ID, (i.e. edit the ID of the current post to display the complete content of another post).

Answer 1

To alter the page ID before the query is run, hook the request filter.

If you're using pretty permalinks, pagename will be set, you can overwrite pagename with another page slug:

function wpd_265903_request( $request ) {
    if( isset( $request['pagename'] ) ){ // any page
        $request['pagename'] = 'some-other-slug';
    }
    return $request;
}
add_filter('request', 'wpd_265903_request');

or you can unset pagename and set page_id:

function wpd_265903_request( $request ) {
    if( isset( $request['pagename'] ) ){
        unset( $request['pagename'] );
        $request['page_id'] = 106;
    }
    return $request;
}
add_filter( 'request', 'wpd_265903_request' );

Answer 2

Yes, it's possible to do something like that. And in fact, if you want to remove actions/filters, then you will be almost certainly be required to use one hook to remove it.

For instance, plugins load before the theme. So if a theme adds a hook to the init hook from the functions.php file like so:

add_action( 'init', 'wpse_265903_init' );

Plugins would not be able to remove that action until after the theme gets setup:

add_action( 'after_setup_theme', function() {
  remove_action( 'init', 'wpse_265903_init' );
} );

Similarly, if you want some hooks to run only after the admin_init hook, then you can do something like this:

add_action( 'admin_init', function() {
  add_action( 'the_post', function() {
    do_something();
  } );
} );

The init hook fires on every request though, so I'm not sure the purpose of adding hooks from that particular hook.