function mgroup($list, $by, ...)

libphutil Technical Documentation (Core Utilities)

Group a list of objects by the result of some method, similar to how GROUP BY works in an SQL query. This function simplifies grouping objects by some property:

$animals_by_species = array();
foreach ($animals as $animal) {
  $animals_by_species[$animal->getSpecies()][] = $animal;
}

This can be expressed more tersely with mgroup():

$animals_by_species = mgroup($animals, 'getSpecies');

In either case, the result is a dictionary which maps species (e.g., like "dog") to lists of animals with that property, so all the dogs are grouped together and all the cats are grouped together, or whatever super businessesey thing is actually happening in your problem domain.

See also igroup(), which works the same way but operates on array indexes.