search for tags at search_hooks.php at mod/search (core plugin of elgg)

search_tag_hook() at modsearchsearch_hooks.php

<?php


/** copied a part of code: /mod/search/search_hooks.php
 * Return default results for searches on tags.
 *
 * @param unknown_type $hook
 * @param unknown_type $type
 * @param unknown_type $value
 * @param unknown_type $params
 * @return unknown_type
 */
function search_tags_hook($hook, $type, $value, $params) {
	$db_prefix = elgg_get_config('dbprefix');

	$valid_tag_names = elgg_get_registered_tag_metadata_names();

	// @todo will need to split this up to support searching multiple tags at once.
	$query = sanitise_string($params['query']);

	// if passed a tag metadata name, only search on that tag name.
	// tag_name isn't included in the params because it's specific to
	// tag searches.
	if ($tag_names = get_input('tag_names')) {
		if (is_array($tag_names)) {
			$search_tag_names = $tag_names;
		} else {
			$search_tag_names = array($tag_names);
		}

		// check these are valid to avoid arbitrary metadata searches.
		foreach ($search_tag_names as $i => $tag_name) {
			if (!in_array($tag_name, $valid_tag_names)) {
				unset($search_tag_names[$i]);
			}
		}
	} else {
		$search_tag_names = $valid_tag_names;
	}

	if (!$search_tag_names) {
		return array('entities' => array(), 'count' => $count);
	}

	// don't use elgg_get_entities_from_metadata() here because of
	// performance issues.  since we don't care what matches at this point
	// use an IN clause to grab everything that matches at once and sort
	// out the matches later.
	$params['joins'][] = "JOIN {$db_prefix}metadata md on e.guid = md.entity_guid";
	$params['joins'][] = "JOIN {$db_prefix}metastrings msn on md.name_id = msn.id";
	$params['joins'][] = "JOIN {$db_prefix}metastrings msv on md.value_id = msv.id";

	$access = get_access_sql_suffix('md');
	$sanitised_tags = array();

	foreach ($search_tag_names as $tag) {
		$sanitised_tags[] = '"' . sanitise_string($tag) . '"';
	}

	$tags_in = implode(',', $sanitised_tags);

	$params['wheres'][] = "(msn.string IN ($tags_in) AND msv.string = '$query' AND $access)";

	$params['count'] = TRUE;
	$count = elgg_get_entities($params);

	// no need to continue if nothing here.
	if (!$count) {
		return array('entities' => array(), 'count' => $count);
	}
	
	$params['count'] = FALSE;
	$entities = elgg_get_entities($params);

	// add the volatile data for why these entities have been returned.
	foreach ($entities as $entity) {
		$matched_tags_strs = array();

		// get tags for each tag name requested to find which ones matched.
		foreach ($search_tag_names as $tag_name) {
			$tags = $entity->getTags($tag_name);

			// @todo make one long tag string and run this through the highlight
			// function.  This might be confusing as it could chop off
			// the tag labels.
			if (in_array(strtolower($query), array_map('strtolower', $tags))) {
				if (is_array($tags)) {
					$tag_name_str = elgg_echo("tag_names:$tag_name");
					$matched_tags_strs[] = "$tag_name_str: " . implode(', ', $tags);
				}
			}
		}

		// different entities have different titles
		switch($entity->type) {
			case 'site':
			case 'user':
			case 'group':
				$title_tmp = $entity->name;
				break;

			case 'object':
				$title_tmp = $entity->title;
				break;
		}

		// Nick told me my idea was dirty, so I'm hard coding the numbers.
		$title_tmp = strip_tags($title_tmp);
		if (elgg_strlen($title_tmp) > 297) {
			$title_str = elgg_substr($title_tmp, 0, 297) . '...';
		} else {
			$title_str = $title_tmp;
		}

		$desc_tmp = strip_tags($entity->description);
		if (elgg_strlen($desc_tmp) > 297) {
			$desc_str = elgg_substr($desc_tmp, 0, 297) . '...';
		} else {
			$desc_str = $desc_tmp;
		}

		$tags_str = implode('. ', $matched_tags_strs);
		$tags_str = search_get_highlighted_relevant_substrings($tags_str, $params['query']);

		$entity->setVolatileData('search_matched_title', $title_str);
		$entity->setVolatileData('search_matched_description', $desc_str);
		$entity->setVolatileData('search_matched_extra', $tags_str);
	}

	return array(
		'entities' => $entities,
		'count' => $count,
	);
}