PHP multidimensional array search by value

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.