In order to use the php file_exists() function you need the server root relative path. The wp_upload_dir() function provides the path to the uploads directory. You can then use that path in combination with many php functions to achive your goal. This snippet creates a new directory called "myNewDirectory" if it does not exist.
$uploads = wp_upload_dir();
$wp_upload_directory = $uploads['basedir'];
//Check if quantum uploads exists and create
if (file_exists($wp_upload_directory . "/myNewDirectory")){
//Do Nothing
}
else {
mkdir($wp_upload_directory . "/myNewDirectory", 0777);
}
