3v4l.org

run code in 300+ PHP versions simultaneously
<?php ini_set('error_reporting', E_ALL); ini_set('display_errors', 1); $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 - 4.3.11, 4.4.0 - 4.4.9, 5.0.0 - 5.0.5, 5.1.0 - 5.1.6, 5.2.0 - 5.2.17, 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
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) } }

preferences:
280.4 ms | 406 KiB | 456 Q