Provide the default form for setting options.
Overrides ArgumentDefaultPluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['code'] = array(
'#type' => 'textarea',
'#title' => t('PHP contextual filter code'),
'#default_value' => $this->options['code'],
'#description' => t('Enter PHP code that returns a value to use for this filter. Do not use <?php ?>. You must return only a single value for just this filter. Some variables are available: the view object will be "$view". The argument handler will be "$argument", for example you may change the title used for substitutions for this argument by setting "argument->validated_title"".'),
);
// Only do this if using one simple standard form gadget
$this
->check_access($form, 'code');
}