class method as callback function

Hi,

I am trying to use a method declared in a parent class as
callback for let's say an "array_walk" function.
I have found some information here
http://www.faqts.com/knowledge_base/...id/1725/fid/39

/*$this->"print_array" does the job...
DZ: Or, a better method:
array_walk($fruits, array($this, 'print_array'));
*/
The second method warks although I do not see how it can
be justified after reading the documentation.
How can I use such a static method as callback function ?
I am using PHP5 of course.

Johan