gistfile1.php

<?php

function my_searchwp_include_only_category( $ids, $engine, $terms ) {

	// if and only if a category ID was submitted:
	// we only want to apply this limitation to the default search engine
	// if you would like to change that you can do so here
	if ( ! empty( $_GET['swp_category_limiter'] ) && 'default' == $engine ) {

		$category_id = absint( $_GET['swp_category_limiter'] );

		$category_args = array(
				'category'       => $category_id,  // limit to the chosen category ID
				'fields'         => 'ids',         // we only want the IDs of these posts
				'posts_per_page' => -1,            // return ALL posts
			);

		$ids = get_posts( $category_args );

		// if there were no posts returned we need to force an empty result
		if ( 0 == count( $ids ) ) {
			$ids = array( 0 ); // this will force SearchWP to return zero results
		}

	}

	// always return our $ids
	return $ids;

}

add_filter( 'searchwp_include', 'my_searchwp_include_only_category', 10, 3 );