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: members 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)