3v4l.org

run code in 300+ PHP versions simultaneously
<?php $a = array('a'=>1,'b'=>2,'c'=>3); var_dump(array_reduce($a, function($result, $item) use ($a) { $key = $result ? key($result) : $a; $key = $key == 'a' ? 'aa' : $key; $result[$key] = $item; return $result; }));

preferences:
39.18 ms | 402 KiB | 5 Q