#key - array_search - recherche dans un tableau multidimensionnel
in (PHP 5 >= 5.5.0) you don't have to write your own function to search through a multi dimensional array

ex : 

$userdb=Array
(
    (0) => Array
        (
            (uid) => '100',
            (name) => 'Sandra Shush',
            (url) => 'urlof100'
        ),

    (1) => Array
        (
            (uid) => '5465',
            (name) => 'Stefanie Mcmohn',
            (pic_square) => 'urlof100'
        ),

    (2) => Array
        (
            (uid) => '40489',
            (name) => 'Michael',
            (pic_square) => 'urlof40489'
        )
);

simply u can use this

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