$all_items = [
  $a1 = [ 'name' => 'item1', 'description' => 'item 1', 'item_key' => 123 ],
  $a2 = [ 'name' => 'item2', 'description' => 'item 2', 'item_key' => 456 ],
  $a3 = [ 'name' => 'item3', 'description' => 'item 3', 'item_key' => 789 ]
];

$invoice_items = [
  $b1 = [ 'item_key' => 123, 'desc' => '1item' ],
  $b2 = [ 'item_key' => 456, 'desc' => '2item' ],
  $b3 = [ 'item_key' => 123, 'desc' => '3item' ],
  $b4 = [ 'item_key' => 345, 'desc' => '4item' ],
  $b5 = [ 'item_key' => 123, 'desc' => '5item' ],
  $b6 = [ 'item_key' => 345, 'desc' => '6item' ],
  $b7 = [ 'item_key' => 123, 'desc' => '7item' ]
];

$invoices = [];
foreach ($invoice_items as $invoice_item) {
  $invoices[$invoice_item['item_key']][] = $invoice_item;
}

$result = [];
foreach ($all_items as $all_item) {
  $result[] = array_merge([ $all_item ], $invoices[$all_item['item_key']] ?? []);
}

結果數組的結構與您在上面的問題中描述的相同。

但是您的結果集與您的註釋“...將invoice_item數字組合並到新數組中”之間存在矛盾。您的結果數組附加了發票項目，因此我上面的代碼。

現在，這個解決方案確實會創建一個新的鍵 - 這裡稱為“descs” - 在這個鍵下的發票項目中的一個數組：

$invoices = [];
foreach ($invoice_items as $invoice_item) {
  $invoices[$invoice_item['item_key']][] = $invoice_item;
}

foreach ($all_items as &$all_item) {
  $all_item['descs'] = $invoices[$all_item['item_key']] ?? [];
}
unset($all_item);

由於這些「descs」陣列條目中的item_key是多餘的，您可以將它們刪除並只建立一個字串陣列：

$invoices = [];
foreach ($invoice_items as $invoice_item) {
  $invoices[$invoice_item['item_key']][] = $invoice_item['desc'];
}

foreach ($all_items as &$all_item) {
  $all_item['descs'] = $invoices[$all_item['item_key']] ?? [];
}
unset($all_item);

輸出：

Array
(
    [0] => Array
        (
            [name] => item1
            [description] => item 1
            [item_key] => 123
            [descs] => Array
                (
                    [0] => 1item
                    [1] => 3item
                    [2] => 5item
                    [3] => 7item
                )

        )

    [1] => Array
        (
            [name] => item2
            [description] => item 2
            [item_key] => 456
            [descs] => Array
                (
                    [0] => 2item
                )

        )

    [2] => Array
        (
            [name] => item3
            [description] => item 3
            [item_key] => 789
            [descs] => Array
                (
                )

        )

)

請注意，您的輸入數組都在其構造中有賦值語句（$a1 = ...，$a2 = ...等）。這沒有太多意義，除非您以後需要這些變數。如果是這樣，編碼方式更可讀性更好：

$a1 = [ 'name' => 'item1', 'description' => 'item 1', 'item_key' => 123 ],
$a2 = [ 'name' => 'item2', 'description' => 'item 2', 'item_key' => 456 ],
$a3 = [ 'name' => 'item3', 'description' => 'item 3', 'item_key' => 789 ]

$all_items = [ $a1, $a2, $a3 ];