Trouble interpreting error message

Hello; I am having trouble interpreting this error message Fatal error: Function name must be a string in (path)/dev_lab_current/lab_15/index.php on line 27 I have tried various variations such as function test()

              {
               // etc...
              }

This is just experimental. Does this mean that anonymous function code cannot be assigned to an associative array items? Here is the code: class _DEV

      {
       private static $_className = '';
       private static $_runList = array();
       public function __construct()
              {
               self::$_className = get_class($this);
              }

       public function run($_what, $_code, $_params)
              {
               $_errHead = self::$_className." -> ".__FUNCTION__;
               $_out = array();
               self::$_runList[$_what] = $_code;
               $_out = self::$_runList[$_what]($_params); // >>> line # 27 referred to in error message
               return $_out;
              }
      }

$_trial = new _DEV(); $_test = function()

         {
          $_out = array();
          $_out['passed'] = "Appears to be working";
          return $_out;
         };

$_result = $_trial->run('test', $_test, '');