CODE:

/**
 * List files and folders inside a directory into a deep array.
 *
 * @param string $Path
 * @return array/null
 */
function EnumFiles($Path){
    // Validate argument
    if(!is_string($Path) or !strlen($Path = trim($Path))){
        trigger_error('$Path must be a non-empty trimmed string.', E_USER_WARNING);
        return null;
    }
    // If we get a file as argument, resolve its folder
    if(!is_dir($Path) and is_file($Path)){
        $Path = dirname($Path);
    }
    // Validate folder-ness
    if(!is_dir($Path) or !($Path = realpath($Path))){
        trigger_error('$Path must be an existing directory.', E_USER_WARNING);
        return null;
    }
    // Store initial Path for relative Paths (second argument is reserved)
    $RootPath = (func_num_args() > 1) ? func_get_arg(1) : $Path;
    $RootPathLen = strlen($RootPath);
    // Prepare the array of files
    $Files = array();
    $Iterator = new DirectoryIterator($Path);
    foreach($Iterator as /** @var \SplFileInfo */ $File){
        if($File->isDot()) continue; // Skip . and ..
        if($File->isLink() or (!$File->isDir() and !$File->isFile())) continue; // Skip links & other stuff
        $FilePath = $File->getPathname();
        $RelativePath = str_replace('\\', '/', substr($FilePath, $RootPathLen));
        $Files[$RelativePath] = $FilePath; // Files are string
        if(!$File->isDir()) continue;
        // Calls itself recursively [regardless of name :)]
        $SubFiles = call_user_func(__FUNCTION__, $FilePath, $RootPath);
        $Files[$RelativePath] = $SubFiles; // Folders are arrays
    }
    return $Files; // Return the tree
}

Test its output and figure it out :) You can do it!