unfortunately the function is_subclass_of does not accept class names in both arguments like get_parent_class since 4.0.5. it would be nice if you could use is_subclass_of in the following way:

is_subclass_of($foo,"bar")
  true if $foo is an object of a subclass of bar
  (this is how it works now)

is_subclass_of("foo","bar")
  true if foo is a subclass of bar
  (a wrote a function to do this, see my note on is_subclass_of) 

is_subclass_of("foo",$bar)
  true if foo is a subclass of the class $bar
  = is_subclass_of("foo",get_class($bar));

is_subclass_of($foo,$bar)
  true if $foo is an object of the class of $bar

thanx a lot!

Jakob