Change layout of views page based on taxonomy term field I have a taxonomy term page being displayed via a Views 'page' display. I need to change a value in the header of the site (above the level of the view) based on the value of one of the fields in the taxonomy term. 

The problem is that I can't find the right hook for this. If I hook into hook_preprocess_views_view, it's too late. The wrapper that includes the site header has already fired. If I try to run hook_preprocess_page, there isn't yet any information in the $variables arg to give me the taxonomy ID so I can look up the field value. Unless there is are variables available to the hook in addition those that are passed into function as args, I don't see how I can get the requisite information. Are there other variables I could use that are undocumented or global variables available that just aren't passed to the function/hook?
What you can do is in the hook_preprocess_page use the following code :

$arg = arg();
if (count($arg) == 3 &amp;&amp; $arg[0] == 'taxonomy' &amp;&amp; $arg[1] == 'term' &amp;&amp; is_numeric($arg[2])) {
  $term = taxonomy_term_load($arg[2]);
  if ($term-&gt;vid == $my_vocab_id) {
    # code...
  }
}


Then change the value of the header as you want in the # code... part.

Basically you check if the URL is a taxonomy term page and then  you can check that the term vid correspond to its proper vocabulary id.

Hope that will help you.