3v4l.org

run code in 300+ PHP versions simultaneously
<?php $a = array('a' => 10, 'z' => 10, 'c' => 10, 'b' => 11, 'd' => 10); $b = array(); foreach($a as $k => $v) $b[$v][] = $k; ksort($b); $a = array(); foreach($b as $k => $v) { foreach($v as $i) $a[$i] = $k; } var_dump($a);

preferences:
32.28 ms | 402 KiB | 5 Q