d and written to a file with a
	 * header indicating the expiration time. File locking can be used for write operations if enabled.
	 *
	 * @param string $key        An MD5 of the DB query.
	 * @param mixed  $content    Data to be cached.
	 * @param int    $expiration Optional. Time in seconds until the cache entry expires. Default is 0 (no expiration).
	 * @param string $group      Optional. The group to which the cache belongs. Default is an empty string.
	 *
	 * @return bool True on success, false on failure.
	 */
	public function set( $key, $content, $expiration = 0, $group = '' ) {
		/**
		 * Get the file pointer of the cache file.
		 * The $key is transformed to a storage key (format "w3tc_INSTANCEID_HOST_BLOGID_dbcache_HASH").
		 * The file path is in the format: CACHEDIR/db/BLOGID/GROUP/[0-9a-f]{3}/[0-9a-f]{3}/[0-9a-f]{32}.
		 */
		$fp = $this->fopen_write( $key, $group, 'wb' );

		if ( ! $fp ) {
			return false;
		}

		if ( $this->_locking ) {
			@flock( $fp, LOCK_EX );
		}

		if ( $expiration <= 0 || $expiration > W3TC_CACHE_FILE_EXPIRE_MAX ) {
			$expiration = W3TC_CACHE_FILE_EXPIRE_MAX;
		}

		$expires_at = time() + $expiration;
		@fputs( $fp, pack( 'L', $expires_at ) );
		@fputs( $fp, '