Form element validation handler for #type 'url'.
Note that #maxlength and #required is validated by _form_validate() already.
function form_validate_url(&$element, &$form_state) {
$value = trim($element['#value']);
form_set_value($element, $value, $form_state);
if ($value !== '' && !valid_url($value, TRUE)) {
form_error($element, t('The URL %url is not valid.', array(
'%url' => $value,
)));
}
}