If you are using (PHP 5 >= 5.5.0) you don't have to write your own function to do this, just write this line and it's done.

If you want just one result:

$key = array_search(40489, array_column($userdb, 'uid'));

For multiple results

$keys = array_keys(array_column($userdb, 'uid'), 40489);

In case you have an associative array as pointed in the comments you could make it with:

single result :

$key = array_search(40489, array_combine(array_keys($userdb), array_column($userdb, 'uid')))

Multiple results :

$keys = array_keys(array_combine(array_keys($userdb), array_column($userdb, 'uid')),40489);

If you are using PHP < 5.5.0, you can use this backport, thanks ramsey!

Update: I've been making some simple benchmarks and the multiple results form seems to be the fastest one, even faster than the Jakub custom function!

You can iterate that objects:

function findObjectById($id){
    $array = array( /* your array of objects */ );

    foreach ( $array as $element ) {
        if ( $id == $element->id ) {
            return $element;
        }
    }

    return false;
}

Faster way is to have an array with keys equals to objects' ids (if unique);

Then you can build your function as follow:

function findObjectById($id){
    $array = array( /* your array of objects with ids as keys */ );

    if ( isset( $array[$id] ) ) {
        return $array[$id];
    }

    return false;
}

Here's a basic solution

function array_find($xs, $f) {
  foreach ($xs as $x) {
    if (call_user_func($f, $x) === true)
      return $x;
  }
  return null;
}

array_find([1,2,3,4,5,6], function($x) { return $x > 4; });  // 5
array_find([1,2,3,4,5,6], function($x) { return $x > 10; }); // null

In the event $f($x) returns true, the loop short circuits and $x is immediately returned. Compared to array_filter, this is better for our use case because array_find does not have to continue iterating after the first positive match has been found.

In the event the callback never returns true, a value of null is returned.

Note, I used call_user_func($f, $x) instead of just calling $f($x). This is appropriate here because it allows you to use any compatible callable

Class Foo {
  static private $data = 'z';
  static public function match($x) {
    return $x === self::$data;
  }
}

array_find(['x', 'y', 'z', 1, 2, 3], ['Foo', 'match']); // 'z'

Of course it works for more complex data structures too

$data = [
  (object) ['id' => 1, 'value' => 'x'],
  (object) ['id' => 2, 'value' => 'y'],
  (object) ['id' => 3, 'value' => 'z']
];

array_find($data, function($x) { return $x->id === 3; });
// stdClass Object (
//     [id] => 3
//     [value] => z
// )

If you're using PHP 7, add some type hints

function array_find(array $xs, callable $f) { ...

If your array may contain null elements, array_find cannot return null to signal no element was not found. As @dossy suggests, you could use an array result containing either one or zero elements -

function array_find($xs, $f) {
  foreach ($xs as $x) {
    if (call_user_func($f, $x) === true)
      return [$x]; // result
  }
  return []; // not found
}

array_find([1,2,3,4,5,6], function($x) { return $x > 4; });  // [5]
array_find([1,2,3,4,5,6], function($x) { return $x > 10; }); // []