How create a new variable according field value
Your code is already there, i think you didn't extract the value of the field_display_reviews correctly, try the following:

 /**
 * Implements hook_preprocess_HOOK().
 */
function mytemplate_preprocess_page(&amp;$variables) {
  if (isset($variables['node'])) {
    $node = $variables['node'];
    if ($node-&gt;type == 'config') {
      if (
        isset($node-&gt;field_display_reviews[LANGUAGE_NONE][0]['value'])
        &amp;&amp; $node-&gt;field_display_reviews[LANGUAGE_NONE][0]['value'] == 'yes'
      ) {
        $variables['display_reviews'] = 'this variable need be created';
      }
    }
  }
}


NOTE: if your display_reviews field of type Boolean the value it will be 1 instead of yes, print the value of $node-&gt;field_display_reviews[LANGUAGE_NONE][0]['value'] before if to see.