What I don't understand about the config_item function in the CodeIgniter source code

<code class="php">if ( ! function_exists('get_config')) { /** * Loads the main config.php file * * This function lets us grab the config file even if the Config class * hasn't been instantiated yet * * @param array * @return array */ function &get_config(Array $replace = array()) { static $config; if (empty($config)) { $file_path = APPPATH.'config/config.php'; $found = FALSE; if (file_exists($file_path)) { $found = TRUE; require($file_path); } // Is the config file in the environment folder? if (file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/config.php')) { require($file_path); } elseif ( ! $found) { set_status_header(503); echo 'The configuration file does not exist.'; exit(3); // EXIT_CONFIG } // Does the $config array exist in the file? if ( ! isset($config) OR ! is_array($config)) { set_status_header(503); echo 'Your config file does not appear to be formatted correctly.'; exit(3); // EXIT_CONFIG } } // Are any values being dynamically added or replaced? foreach ($replace as $key => $val) { $config[$key] = $val; } return $config; } } // ------------------------------------------------------------------------ if ( ! function_exists('config_item')) { /** * Returns the specified config item * * @param string * @return mixed */ function config_item($item) { static $_config; if (empty($_config)) { // references cannot be directly assigned to static variables, so we use an array $_config[0] =& get_config(); } return isset($_config[0][$item]) ? $_config[0][$item] : NULL; } }</code>

<code class="php">if ( ! function_exists('get_config')) { /** * Loads the main config.php file * * This function lets us grab the config file even if the Config class * hasn't been instantiated yet * * @param array * @return array */ function &get_config(Array $replace = array()) { static $config; if (empty($config)) { $file_path = APPPATH.'config/config.php'; $found = FALSE; if (file_exists($file_path)) { $found = TRUE; require($file_path); } // Is the config file in the environment folder? if (file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/config.php')) { require($file_path); } elseif ( ! $found) { set_status_header(503); echo 'The configuration file does not exist.'; exit(3); // EXIT_CONFIG } // Does the $config array exist in the file? if ( ! isset($config) OR ! is_array($config)) { set_status_header(503); echo 'Your config file does not appear to be formatted correctly.'; exit(3); // EXIT_CONFIG } } // Are any values being dynamically added or replaced? foreach ($replace as $key => $val) { $config[$key] = $val; } return $config; } } // ------------------------------------------------------------------------ if ( ! function_exists('config_item')) { /** * Returns the specified config item * * @param string * @return mixed */ function config_item($item) { static $_config; if (empty($_config)) { // references cannot be directly assigned to static variables, so we use an array $_config[0] =& get_config(); } return isset($_config[0][$item]) ? $_config[0][$item] : NULL; } }</code>