kartik\select2\Select2::initPlaceholder PHP Method

    protected function initPlaceholder()
    {
        $isMultiple = ArrayHelper::getValue($this->options, 'multiple', false);
        if (isset($this->options['prompt']) && !isset($this->pluginOptions['placeholder'])) {
            $this->pluginOptions['placeholder'] = $this->options['prompt'];
            if ($isMultiple) {
                unset($this->options['prompt']);
            }
            return;
        }
        if (isset($this->options['placeholder'])) {
            $this->pluginOptions['placeholder'] = $this->options['placeholder'];
            unset($this->options['placeholder']);
        }
        if (isset($this->pluginOptions['placeholder']) && is_string($this->pluginOptions['placeholder']) && !$isMultiple) {
            $this->options['prompt'] = $this->pluginOptions['placeholder'];
        }
    }