unique_mapping() - create a mapping from an array based on a function
mapping unique_mapping( mixed *arr, string fun, object ob,
mixed extra, ... );
mapping unique_mapping( mixed *arr, function f, mixed extra, ... );
Returns a mapping built in the following manner:
'ob->fun()' or 'f' is evaluated for each member of the array. The mem‐
bers for which the function returns the same value are grouped
together, and associated with the return value as the key.
The key/value ordering in the returned mapping is consistent but not
guaranteed to not change in the future.
filter_array(3), sort_array(3), map(3)