How to merge values of two arrays?

I have two ar开发者_Go百科rays:

$a = array(
    array("id" => 132),
    array("id" => 131),
    array("id" => 129),
    array("id" => 128)
)

$b = array(
    array("id" => 129, "attachment_id" => 111),
    array("id" => 129, "attachment_id" => 112),
    array("id" => 132, "attachment_id" => 113),
    array("id" => 132, "attachment_id" => 114),
    array("id" => 132, "attachment_id" => 115)
)

I need to make one array from those two. id key must be unique so it can't be repeated. Problem is that: I need to make new element in that new array called attachments that's array and contains values of attachment_id for that id. You see, there may be more that one attachment_id for id.

Its hard to explain, but I hope that you'll understand! Thanks in advice.

Edit:

Here is desired outcome:

$c = array(
    array("id" => 132, 'attachments' => array(113, 114, 115)),
    array("id" => 131, 'attachments' => array()),
    array("id" => 129, 'attachments' => array(111, 112)),
    array("id" => 128, 'attachments' => array())
);

---

Something like:

$a = array(
    array("id" => 132),
    array("id" => 131),
    array("id" => 129),
    array("id" => 128)
);

$b = array(
    array("id" => 129, "attachment_id" => 111),
    array("id" => 129, "attachment_id" => 112),
    array("id" => 132, "attachment_id" => 113),
    array("id" => 132, "attachment_id" => 114),
    array("id" => 132, "attachment_id" => 115)
);

$c = array();

foreach ($a as $idEntry) {
    $tmp = array(
        'id' => $idEntry['id'],
        'attachments' => array()
    );

    foreach ($b as $bIdx => $attachmentEntry) {
        if ($attachmentEntry['id'] == $idEntry['id']) {
            $tmp['attachments'][] = $attachmentEntry['attachment_id'];
            unset($b[$bIdx]);
        }
    }

    $c[] = $tmp;
}

print_r($c);

---

Something like this should do it:

# Temporary array
$c = array();

# Loop through all attachments and index them by id
foreach($b as $row) {
    if(!isset($c[$row['id']])) {
        $c[$row['id']] = array();
    }
    $c[$row['id']][] = $row['attachment_id'];
}

# Loop through ids and add attachments where available
foreach($a as &$row) {
    $row['attachments'] = isset($c[$row['id']]) ? $c[$row['id']] : array()
} unset($row);

Output:

array(
    array("id" => 132, "attachments" => array(113, 114, 115)),
    array("id" => 131, "attachments" => array()),
    array("id" => 129, "attachments" => array(111, 112)),
    array("id" => 128, "attachments" => array())
)

---

Index your arrays by 'id' and then use http://php.net/manual/en/function.array-merge.php