gistfile1.php

<?php 
class BlogHolder_ControllerExtension extends DataExtension {

	public static $allowed_actions = array('ArticleSearchForm', 'search');

	public function ArticleSearchForm() {
		// Create fields

		$categories = array();
		foreach($this->owner->BlogCategories() as $category){
			$categories[$category->ID] = $category->Title;
		}

		$keywordField = new TextField('Keyword', 'Keywords:');
		$keywordField->setAttribute('placeholder', 'Keywords');

		$fields = new FieldList(
			$keywordField,
			new DropdownField('Category', 'Category:', $categories)
		);

		// Create actions
		$actions = new FieldList(
			new FormAction('search', 'Search')
		);

		return new ArticleSearchForm($this->owner, 'search', $fields, $actions);
	}

	public function search($data, $form){
		die('<pre>');
		var_dump($data, $form);
		die('</pre>');
	}

}