check file existence from root

I tried the following code to check existence of file in root.

if($res['profile_picture']!="" && file_exists("images/".$res['users_id']."/thumnails/".$res['profile_picture'])){
    $photo_p="images/".$res['users_id']."/thumnails/".$res['profile_picture'];
}

It works only on root directory not sub directory.

I'm not sure whether function file_exist checks for both absolute and relative paths so I tried adding ROOT and $_SERVER['DOCUMENT_ROOT']. But still it didn't worked out.

Any Help?