Optimize reordering and duplicate removal of multi-dimensional array

I wondering wheth开发者_C百科er anyone has any good ideas on optimizing the following code. I have an multi-dimensional array ($List) as follows:

Array
(
    [0] => Array
    (
        [id] => 1
        [title] => A good read
        [priority] => 10
    )

    [1] => Array
    (
        [id] => 2
        [title] => A bad read
        [priority] => 20

    )

    [2] => Array
    (
        [id] => 3
        [title] => A good read
        [priority] => 10
    )
)

First I'm removing any entries that share the same title (no matter what the other values are) as follows:

$List_new = array();
foreach ($List as $val) {
    $List_new[$val['title']] = $val;    
}
$List = array_values($List_new);

Perfect. Then I'm reordering the array, first by the priority field and then id:

$sort_id = array();
$sort_priority = array();
foreach ($List as $key => $row) {
    $sort_id[$key] = $row['id'];
    $sort_priority[$key] = $row['priority'];
}
array_multisort($sort_priority, SORT_DESC, $sort_id, SORT_DESC, $List);

Both code blocks appear in a loop, hence the clearing of $sort_id and $sort_priority before reordering.

Is there a better way to do this - i.e. use the sorting process to remove duplicate title entries? This code block is being executed in a loop of up to 500,000 records and so any improvement would be welcome!

One loop, but a few extra function calls so I can't tell you how the Big O changes. One thing to note, the padding around numbers must be big enough to prevent overflow i.e. 2 = max 99 priorities and 6 = max 999,999 items.

$list_titles = array();
foreach($List as $val) {
    if(isset($list_titles[$val['title']])) continue;
    $list_titles[$val['title']] = true;
    $List_new[str_pad($val['priority'], 2, 0, STR_PAD_LEFT).str_pad($val['id'], 6, 0, STR_PAD_LEFT)] = $val;
}
krsort($List_new);

Edit: made some minor modifications.