mixed *filter_objects(mixed *arr, string fun, object ob, mixed extra);

Returns an array holding the items of 'arr' filtered through ob->fun().
The function 'fun' in 'ob' is called for each element in 'arr' with that
element as parameter. A second parameter 'extra' is sent in each call if
given. If ob\->fun(arr[.index.],\ extra) returns 1 the element is included in
the returned array. 

If 'arr' is not an array, then 0 will be returned.
