- array_splice: documentation ( source)
- array_search: documentation ( source)
- var_export: documentation ( source)
- array_keys: documentation ( source)
<?php
$arr = [1 => 'monkey', 2 => 'giraffe', 3 => 'lion', 4 => 'walrus', 5 => 'zebra', 6 => 'narwhal'];
$arr += array_splice($arr, array_search('walrus', array_keys($arr)), 1);
var_export($arr);