Array inside array

You will need to foreach $items_list_result making a new array with a reference too "invoice_no". And later on append it to $invoice_details_result with another foreach.

Code:

$children = [];
foreach($items_list_result as $l)
{
    $children[$l['invoice_no']][] = $l;
}

foreach($invoice_details_result as $l)
{
    $l['line_items'] = isset($children[$l['invoice_no']]) ? $children[$l['invoice_no']] : [];
}

This line should be removed:
$particular_customers_sales_order = array_merge($invoice_details_result,$items_list_result);