Making an add_filter() call from within an add_filter() call

Question

I am creating a plugin where I am checking for parameters passed in the url for a page, then using the values of those parameters to change the Title, meta tags and make some other SEO changes. But I am running into problems trying to combine actions and filters properly to make this happen. Here is a sample of some of the things I am doing. First I am registering the variable passed in the url

    add_action('init', 'register_tag');

    function register_tag() {
      add_rewrite_tag('%address%','([^&]+)');
    }

Then I am obtaining the variable from $wp_query and setting the Title amongst other things.

    add_filter('wp_head', 'seo_features');

    function seo_features() {
       global $wp_query,$post,$address;
       $address = $wp_query->query_vars['address'];
       if ($address) {
          $post->post_title = $address;
          add_filter('wp_title', set_title);
          add_filter('wpseo_title', set_title);
          remove_action('wp_head', 'rel_canonical');
       }
    }
    function set_title($title) {
       global $address;
       return $address;
    }

But I don't think you can make add_action() or add_filter() calls from within an add_filter()?

I'm not sure how to do this correctly. I have to use add_action("init") to register the tag (and also to add a related rewrite rule using add_rewrite_rule() ). But it seems I can only get the value of the variable $address from $wp_query from within an add_filter call. Trying to access it without a filter/action doesn't work as it is called too early.

Any ideas on what the correct order I should be doing this?

Answer 1

You can add or remove hooks from inside other hooks if you get the timing right but I don't understand why you are making this so complex.

function set_title($title) {
  global $wp_query,$post,$address;
  $address = $wp_query->query_vars['address'];
  if ($address) {
    return $address;
  }
}
add_filter('wp_title', 'set_title');

If you need the share the value, use a static variable:

function my_address() {
  static $address = '';
  global $wp_query;
  if (empty($address)) {
    $address = $wp_query->query_vars['address'];
  }
  return $address;
}

function set_title($title) {
  return my_address();
}
add_filter('wp_title', 'set_title');

Or wrap the whole thing in a class.