- array_map: documentation ( source)
- array_chunk: documentation ( source)
- array_merge: documentation ( source)
- var_export: documentation ( source)
<?php
$list = ['item1','item2','item3','item4','item5','item6'];
var_export(
array_map(fn($row) => array_merge([999999], $row), array_chunk($list, 2))
);