- array_map: documentation ( source)
- array_flip: documentation ( source)
<?php
$start = 6;
$list = [
'1楼', '2楼', '3楼', '4楼', '5楼',
];
$list = array_flip(
array_map(
array_flip($list),
function($key){ return $start+(int)$key; }
)
);