- var_dump: documentation ( source)
- array_multisort: documentation ( source)
- array_keys: documentation ( source)
<?php
$l = [
'a' => 1,
'b' => 2,
'c' => 2,
'd' => 4,
'e' => 5,
'f' => 5
];
array_multisort($l, SORT_DESC, array_keys($l));
var_dump($l);