How to override $page_top variable?
You override markup by using :

  function template_preprocess_field(&amp;$vars){ 
    if($vars['element']['#field_name'] == 'field_event_geofield')
     {
          //kpr($vars); //Uncomment to output the $vars array.

           if(isset($vars['element']['#items']['0']['lat']) &amp;&amp; isset($vars['element']['#items']['0']['lon'])) { $vars['items']['0']['#markup'] = 'View Map'; } return; } 
           }