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<?php

$ary = [
    ['name' => 'yamada'],
    ['name' => 'tanaka'],
    ['name' => 'komori'],
];

$res = false;

foreach ($ary as $val) {
    $res =  array_search('oomori', $val);
    //   print_r ($val);
    if (!$res === false) {
        break;
    }
}

var_dump($res); // 名前が含まれない場合、falseが返る

<?php
// array_colum使用ver
$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'));
var_dump($key); // => int(2)