Simple web-based Shell class

<?php
    class Shell {
        /* Define constants */
        // This can be set to null for unrestricted buffer space
        const MAX_OUTPUT_BUFFER = 256;

        /* Define static properties */
        private static $output_buffer = array();

        /**
         * Execute a command, add the output to the buffer stack
         * @return Int Command return status
         * @param $command String
         */
        static function exec($command) {
            @exec($command, $tmp_ob, $status);
            Shell::$output_buffer = array_merge((array) Shell::$output_buffer, (array) $tmp_ob);
            while(count(Shell::$output_buffer) > Shell::MAX_OUTPUT_BUFFER && Shell::MAX_OUTPUT_BUFFER != null) {
                array_shift(Shell::$output_buffer);
            }
            return (int) $status;
        }

        /**
         * Flush the buffer stack
         * @return null
         */
        static function flush() {
            Shell::$output_buffer = array();
        }

        /**
         * Print the buffer stack
         * @return null
         */
        static function output() {
            foreach(Shell::$output_buffer as $output_line) {
                printf("%s\n", $output_line);
            }
        }
    }
?>