<?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)
);