<?php
var_export(
collect([(object) [ "grouper" = 1 , "idx" = 1 , "someField"=" a "],
(object) [ "grouper" = 1 , "idx" = 2 , "someField"=" b "],
(object) [ "grouper" = 2 , "idx" = 3 , "someField"=" c "],
(object) [ "grouper" = 3 , "idx" = 4 , "someField"=" d "],
(object) [ "grouper" = 1 , "idx" = 5 , "someField"=" e "],
(object) [ "grouper" = 2 , "idx" = 6 , "someField"=" f "],
(object) [ "grouper" = 1 , "idx" = 7 , "someField"=" g "],
(object) [ "grouper" = 3 , "idx" = 8 , "someField"=" h "],
(object) [ "grouper" = 3 , "idx" = 9 , "someField"=" i "],
(object) [ "grouper" = 2 , "idx" = 10 , "someField"=" j "],
(object) [ "grouper" = 1 , "idx" = 11 , "someField"=" k "],
(object) [ "grouper" = 1 , "idx" = 12 , "someField"=" l "],
(object) [ "grouper" = 2 , "idx" = 13 , "someField"=" m "]])
->groupBy(fn($v, $k) => $v->grouper)
);