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<?php $arry= array("1"=>"20", "2"=>"20", "3"=>"30", "4"=>"40", "5"=>"20"); $results = array(); foreach($arry as $key=>$value) { $results[$value][] = $key; } var_dump($results);
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(3) { [20]=> array(3) { [0]=> int(1) [1]=> int(2) [2]=> int(5) } [30]=> array(1) { [0]=> int(3) } [40]=> array(1) { [0]=> int(4) } }