function searchForId($id, $array) {
   foreach ($array as $key => $val) {
       if ($val['uid'] === $id) {
           return $key;
       }
   }
   return null;
}

This will work. You should call it like this:

$id = searchForId('100', $userdb);

It is important to know that if you are using === operator compared types have to be exactly same, in this example you have to search string or just use == instead ===.

Based on angoru answer. In later versions of PHP (>= 5.5.0) you can use one-liner.

$key = array_search('100', array_column($userdb, 'uid'));

Here is documentation: http://php.net/manual/en/function.array-column.php.

In php 5.5.5 & later versions, you can try this

$array_subjected_to_search =array(
array(
        'name' => 'flash',
        'type' => 'hero'
    ),

array(
        'name' => 'zoom',
        'type' => 'villian'
    ),

array(
        'name' => 'snart',
        'type' => 'antihero'
    )
);
$key = array_search('snart', array_column($array_subjected_to_search, 'name'));
var_dump($array_subjected_to_search[$key]);

Output:

array(2) { ["name"]=> string(5) "snart" ["type"]=> string(8) "antihero" }

working sample : http://sandbox.onlinephpfunctions.com/code/19385da11fe0614ef5f84f58b6dae80bd216fc01

Documentation about array_column can be found here