<?
class A{
      public function do_a(){ return 'a_done';}

      public function do_b(){ return 'b_done';}
}

$System = new A;

$method_a = $System->do_a();
$method_b = $System->do_b();

echo 'this was the result of ' . $method_a();
echo 'this was the result of ' . $method_b();

Fatal error: Call to undefined function a_done() on line 13