- array_map: documentation ( source)
- var_export: documentation ( source)
<?php
$arr1 = [
['gross_value' => '100', 'quantity' => '1'],
['gross_value' => '200', 'quantity' => '1']
];
$arr2 = [
['item_title_id' => '1', 'order_id' => '4'],
['item_title_id' => '2', 'order_id' => '4']
];
var_export(
array_map('array_merge', $arr1, $arr2)
);