WordPress/wp-includes/meta.php

<?php
/**
 * Meta API
 *
 * Functions for retrieving and manipulating metadata
 *
 * @package WordPress
 * @subpackage Meta
 * @since 2.9.0
 */

function add_metadata($meta_type, $object_id, $meta_key, $meta_value, $unique = false) {
	if ( !$meta_type || !$meta_key )
		return false;

	if ( ! $table = _get_meta_table($meta_type) )
		return false;

	global $wpdb;

	$column = esc_sql($meta_type . '_id');

	// expected_slashed ($meta_key)
	$meta_key = stripslashes($meta_key);

	if ( $unique && $wpdb->get_var( $wpdb->prepare(
		"SELECT COUNT(*) FROM $table WHERE meta_key = %s AND $column = %d",
		$meta_key, $object_id ) ) )
		return false;

	$meta_value = maybe_serialize( stripslashes_deep($meta_value) );

	$wpdb->insert( $table, array(
		$column => $object_id,
		'meta_key' => $meta_key,
		'meta_value' => $meta_value
	) );

	wp_cache_delete($object_id, $meta_type . '_meta');

	do_action( "added_{$meta_type}_meta", $wpdb->insert_id, $object_id, $meta_key, $meta_value );

	return true;
}

function update_metadata($meta_type, $object_id, $meta_key, $meta_value, $prev_value = '') {
	if ( !$meta_type || !$meta_key )
		return false;

	if ( ! $table = _get_meta_table($meta_type) )
		return false;

	global $wpdb;

	$column = esc_sql($meta_type . '_id');

	// expected_slashed ($meta_key)
	$meta_key = stripslashes($meta_key);

	if ( ! $meta_id = $wpdb->get_var( $wpdb->prepare( "SELECT meta_id FROM $table WHERE meta_key = %s AND $column = %d", $meta_key, $object_id ) ) )
		return add_metadata($meta_type, $object_id, $meta_key, $meta_value);

	$meta_value = maybe_serialize( stripslashes_deep($meta_value) );

	$data  = compact( 'meta_value' );
	$where = array( $column => $object_id, 'meta_key' => $meta_key );

	if ( !empty( $prev_value ) ) {
		$prev_value = maybe_serialize($prev_value);
		$where['meta_value'] = $prev_value;
	}

	do_action( "update_{$meta_type}_meta", $object_id, $meta_key, $meta_value, $meta_id );

	$wpdb->update( $table, $data, $where );
	wp_cache_delete($object_id, $meta_type . '_meta');

	do_action( "updated_{$meta_type}_meta", $object_id, $meta_key, $meta_value, $meta_id );

	return true;
}

function delete_metadata($meta_type, $object_id, $meta_key, $meta_value = '', $delete_all = false) {
	if ( !$meta_type || !$meta_key || (!$delete_all && ! (int)$object_id) ) 
		return false;
	
	if ( ! $table = _get_meta_table($meta_type) )
		return false;

	global $wpdb;

	$type_column = esc_sql($meta_type . '_id');
	// expected_slashed ($meta_key)
	$meta_key = stripslashes($meta_key);
	$meta_value = maybe_serialize( stripslashes_deep($meta_value) );

	$query = $wpdb->prepare( "SELECT meta_id FROM $table WHERE meta_key = %s", $meta_key );

	if ( !$delete_all )
		$query .= $wpdb->prepare(" AND $type_column = %d", $object_id );

	if ( $meta_value )
		$query .= $wpdb->prepare(" AND meta_value = %s", $meta_value );

	$meta_ids = $wpdb->get_col( $query );
	if ( !count( $meta_ids ) )
		return false;

	$query = "DELETE FROM $table WHERE meta_id IN( " . implode( ',', $meta_ids ) . " )";

	$count = $wpdb->query($query);

	if ( !$count )
		return false;

	wp_cache_delete($object_id, $meta_type . '_meta');

	do_action( "deleted_{$meta_type}_meta", $object_id, $meta_key, $meta_value, $meta_ids );

	return true;
}

function get_metadata($meta_type, $object_id, $meta_key = '', $single = false) {
	if ( !$meta_type )
		return false;

	$meta_cache = wp_cache_get($object_id, $meta_type . '_meta');

	if ( !$meta_cache ) {
		update_meta_cache($meta_type, $object_id);
		$meta_cache = wp_cache_get($object_id, $meta_type . '_meta');
	}

	if ( ! $meta_key )
		return $meta_cache;

	if ( isset($meta_cache[$meta_key]) ) {
		if ( $single ) {
			return maybe_unserialize( $meta_cache[$meta_key][0] );
		} else {
			return array_map('maybe_unserialize', $meta_cache[$meta_key]);
		}
	}

	if ($single)
		return '';
	else
		return array();
}

function update_meta_cache($meta_type, $object_ids) {
	if ( empty( $meta_type ) || empty( $object_ids ) )
		return false;

	if ( ! $table = _get_meta_table($meta_type) )
		return false;

	$column = esc_sql($meta_type . '_id');

	global $wpdb;

	if ( !is_array($object_ids) ) {
		$object_ids = preg_replace('|[^0-9,]|', '', $object_ids);
		$object_ids = explode(',', $object_ids);
	}

	$object_ids = array_map('intval', $object_ids);

	$cache_key = $meta_type . '_meta';
	$ids = array();
	foreach ( $object_ids as $id ) {
		if ( false === wp_cache_get($id, $cache_key) )
			$ids[] = $id;
	}

	if ( empty( $ids ) )
		return false;

	// Get meta info
	$id_list = join(',', $ids);
	$cache = array();
	$meta_list = $wpdb->get_results( $wpdb->prepare("SELECT $column, meta_key, meta_value FROM $table WHERE $column IN ($id_list)",
		$meta_type), ARRAY_A );

	if ( !empty($meta_list) ) {
		foreach ( $meta_list as $metarow) {
			$mpid = intval($metarow[$column]);
			$mkey = $metarow['meta_key'];
			$mval = $metarow['meta_value'];

			// Force subkeys to be array type:
			if ( !isset($cache[$mpid]) || !is_array($cache[$mpid]) )
				$cache[$mpid] = array();
			if ( !isset($cache[$mpid][$mkey]) || !is_array($cache[$mpid][$mkey]) )
				$cache[$mpid][$mkey] = array();

			// Add a value to the current pid/key:
			$cache[$mpid][$mkey][] = $mval;
		}
	}

	foreach ( $ids as $id ) {
		if ( ! isset($cache[$id]) )
			$cache[$id] = array();
	}

	foreach ( array_keys($cache) as $object)
		wp_cache_set($object, $cache[$object], $cache_key);

	return $cache;
}

function _get_meta_table($type) {
	global $wpdb;

	$table_name = $type . 'meta';

	if ( empty($wpdb->$table_name) )
		return false;

	return $wpdb->$table_name;
}
?>