searchwp-customizations.php

<?php

// adds a Categories dropdown to the search form
function my_searchwp_get_search_form_with_categories_dropdown( $form ) {
	ob_start(); ?>
		<form role="search" method="get" class="search-form" action="<?php echo home_url( '/' ); ?>">
			<label>
				<span class="screen-reader-text">Search For</span>
				<input type="search" class="search-field" placeholder="Search..." value="<?php echo esc_attr( get_search_query() ); ?>" name="s" title="Search for:" />
			</label>
			<?php
				// for more information see http://codex.wordpress.org/Function_Reference/wp_dropdown_categories
				$swp_cat_dropdown_args = array(
						'show_option_all'  => __( 'Any Category' ),
						'name'             => 'swp_category_limiter',
					);
				wp_dropdown_categories( $swp_cat_dropdown_args );
			?>
			<input type="submit" class="search-submit" value="Search" />
		</form>
	<?php return ob_get_clean();
}

add_filter( 'get_search_form', 'my_searchwp_get_search_form_with_categories_dropdown' );