mixed *filter_array(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. 
