Reference PHP array by multiple indexes

It appears your fancy solution was only available as of PHP 5.5. You can combine the use of array_search and array_column to fetch your entry in a single line of code:

$items = [
    [
     'unique_id' => 42,
     'url' => 'http://foo.com'
    ],
    [
     'unique_id' => 57,
     'url' => 'http://bar.com'
    ],
    [
     'unique_id' => 36,
     'url' => 'http://example.com'
    ],

];

$bar = $entries[array_search(57, array_column($items, 'unique_id'))];

var_dump($bar);

//outputs
array (size=2)
    'unique_id' => int 57
    'url' => string 'http://bar.com' (length=14)

Try something like this:

function selectByIdOrURL($array, $data) {
    foreach($array as $row) {
       if($row['unique_id'] == $data || $row['url'] == $data) return $row;
    }
    return NULL;
}

$array = array(
           array('unique_id' => 5, 'url' => 'http://blah.com'),
           array('unique_id' => 3, 'url' => 'http://somewhere_else.com')
         );
$found = selectByIdOrURL($array, 5); //array('unique_id' => 5, 'url' => 'http://blah.com')
$nfound = selectByIdOrURL($array, 10); //NULL

Only way I can think of that doesn't involve iterating the array for each search (see Jacob's answer) is to store references to each item in two arrays.

Edit: As the URLs and IDs cannot collide, they may be stored in the same reference array (thanks Matthew)

$items; // array of item objects
        // Use objects so they're implicitly passed by ref

$itemRef = array();

foreach ($items as $item) {
    $itemRef[$item->unique_id] = $item;
    $itemRef[$item->url] = $item;
}

// find by id
$byId = $itemRef[$id];

// find by url
$byUrl = $itemRef[$url];

You could probably encapsulate this nicely using a collection class that implements getById() and getByUrl(). Internally, it could store the references in as many arrays as is necessary.

Of course, what you're essentially doing here is creating indexed result sets, something best left to database management systems.