Как сделать переход по категориям?

<?php
$uri = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);
$uri = urldecode($uri);
if ($uri === '/' || isset($_SERVER['SCRIPT_FILENAME']) || file_exists($_SERVER['DOCUMENT_ROOT'].$uri)) {
	return false;
} else {
	if (file_exists(__DIR__.$uri)) {
		if (is_dir(__DIR__.$uri)) {
			chdir(__DIR__.rtrim($uri, '/'));
			include ($_SERVER["SCRIPT_FILENAME"] = __DIR__.rtrim($uri, '/').'/index.php');
		} else {
			if (substr($uri, -4) === '.php') {
				chdir(__DIR__.dirname($uri));
				include ($_SERVER["SCRIPT_FILENAME"] = __DIR__.$uri);
			} else {
				header('Content-Type: '.mime_type(__DIR__.$uri));
				readfile(__DIR__.$uri);
			}
		}
	} else {
		include ($_SERVER["SCRIPT_FILENAME"] = $_SERVER['DOCUMENT_ROOT'].'/index.php');
	}
}
function mime_type($filename) {
    $mime_types = array(
        'txt' => 'text/plain',
        'htm' => 'text/html',
        'html' => 'text/html',
        'php' => 'text/html',
        'css' => 'text/css',
        'js' => 'application/javascript',
        'json' => 'application/json',
        'xml' => 'application/xml',
        'swf' => 'application/x-shockwave-flash',
        'flv' => 'video/x-flv',
        // images
        'png' => 'image/png',
        'jpe' => 'image/jpeg',
        'jpeg' => 'image/jpeg',
        'jpg' => 'image/jpeg',
        'gif' => 'image/gif',
        'bmp' => 'image/bmp',
        'ico' => 'image/vnd.microsoft.icon',
        'tiff' => 'image/tiff',
        'tif' => 'image/tiff',
        'svg' => 'image/svg+xml',
        'svgz' => 'image/svg+xml',
        // archives
        'zip' => 'application/zip',
        'rar' => 'application/x-rar-compressed',
        'exe' => 'application/x-msdownload',
        'msi' => 'application/x-msdownload',
        'cab' => 'application/vnd.ms-cab-compressed',
        // audio/video
        'mp3' => 'audio/mpeg',
        'qt' => 'video/quicktime',
        'mov' => 'video/quicktime',
        // adobe
        'pdf' => 'application/pdf',
        'psd' => 'image/vnd.adobe.photoshop',
        'ai' => 'application/postscript',
        'eps' => 'application/postscript',
        'ps' => 'application/postscript',
        // ms office
        'doc' => 'application/msword',
        'rtf' => 'application/rtf',
        'xls' => 'application/vnd.ms-excel',
        'ppt' => 'application/vnd.ms-powerpoint',
        // open office
        'odt' => 'application/vnd.oasis.opendocument.text',
        'ods' => 'application/vnd.oasis.opendocument.spreadsheet',
    );
	$ext = explode('.',$filename);
    $ext = strtolower(array_pop($ext));
    if (array_key_exists($ext, $mime_types)) {
        return $mime_types[$ext];
    }
    elseif (function_exists('finfo_open')) {
        $finfo = finfo_open(FILEINFO_MIME);
        $mimetype = finfo_file($finfo, $filename);
        finfo_close($finfo);
        return $mimetype;
    }
    else {
        return 'application/octet-stream';
    }
}

RewriteEngine On
RewriteBase /

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^.*$ index.php [L]