A variable function can be called on a normal function just fine:
------------------------------
function test()
{
   return "test1";
}

$function = "test";
echo $function();
------------------------------
This outputs: test

However trying the same thing on a funciton inside a class will fail:
------------------------------
class Test
{
  function Test()
  {
  }

  function testClass()
  {
    return "test 2";
  }
}

$function = "Test::testClass";
echo $function();
------------------------------

Calling:
  echo Test::testClass();
works fine, however the above code does not.  It should output: test 2

However it returns an error similar to the following:
Fatal error: Call to undefined function: registration::formatdate() in /home/gaza/public_html/fp_php/support/table.php on line 396