function yourextension_civicrm_postProcess($formName, &$form) {

  //Check if user has an active membership of type "General".
  $activeMembership = civicrm_api3('Membership', 'getcount', array(
    'contact_id' => 107,
    'active_only' => 1,
    'membership_type_id' => "General",
  ));

  //Assign a new variable to the template.
  $form->assign('activeMembership', $activeMembership);
}