Check If Folder Exists Is Not Working

I'm trying to get this function to recognize if a folder exists on the server, but it's not working. I've tried is_dir function as well but no luck.

This script is trying to be utilized in a Wordpress plugin, but it's using the php-based function to look if a file/folder exists. Any idea why it's not recognizing that the /docs folder exists? I've verified that $filename is actually pulling up the correct file path on server.

function docs_settings_page() {
    $filename = get_stylesheet_directory_uri()."/docs/";
    if(file_exists($filename)) {
        echo "The folder exists!";
    }
    else {
        echo "Sorry - the file does not exist.";
    }
}

function docs_create_menu() {
    // Do stuff here
}

add_action('admin_menu', 'docs_create_menu');