Optimize output buffer handler

Hi,
In the class named Cachify, you use ob_start with custom output buffer handler: ob_start( 'Cachify::set_cache' );

I suggest you replace the set_cache method with this one:

	/**
	 * Assign the cache
	 *
	 * @since   0.1
	 * @change  2.0
	 *
	 * @param   string $data  Content of the page.
     * @param   int $phase    Bitmask of PHP_OUTPUT_HANDLER_* constants.
     * @return  string        Content of the page.
	 */
	public static function set_cache( $data, $phase ) {

        if ($phase & PHP_OUTPUT_HANDLER_FINAL || $phase & PHP_OUTPUT_HANDLER_END) {
            /* Empty? */
            if (empty($data)) {
                return '';
            }

            /**
             * Filters whether the buffered data should actually be cached
             *
             * @param bool   $should_cache  Whether the data should be cached.
             * @param string $data          The actual data.
             * @param object $method        Instance of the selected caching method.
             * @param string $cache_hash    The cache hash.
             * @param int    $cache_expires Cache validity period.
             *
             * @since 2.3
             *
             */
            $should_cache = apply_filters('cachify_store_item', true, $data, self::$method, self::_cache_hash(), self::_cache_expires());

            /* Save? */
            if ($should_cache) {
                call_user_func(array(
                    self::$method,
                    'store_item',
                ), self::_cache_hash(), self::_minify_cache($data), self::_cache_expires(), self::_signature_details());
            }
        }

		return $data;
	}

Checking the phase parameter helps to leverage processing as the output callback function called multiple times when ob_flush() involved.
Example:
https://sandbox.onlinephpfunctions.com/code/4dc11a69b0da536768f211199dd8c31d1ca50609