How to rewrite URL with PHP variables with htaccess to a normal looking URL?

Here is an example of adding custom rewrite rule to WordPress.
For instance, you have a link
http://website.com/1/ that would like WordPress interprets internally as
http://website.com?tl=1

But for tl you have mentioned, I am not sure what is your settings. So I assume tl is working in your system. Because WP default parameter is p for page id

You may try to use the following filter in your theme functions.php or plugin

• The following only works if permalinks is turned on. Otherwise, no effect.
  It means in settings -> permalinks, if you choose Plain, then WordPress will show the query without pretty URL. It is regarded as turned off.
  Otherwise, if you choose other options like Post Name then WordPress will accept Post Name as URL such as https://examples.com/sample-post/

So, if it is being enabled(or turned on). Then the the rewrite system will be used. And the following filter works with WordPress rewrite system.

Sometimes we use some other synonym like Enable permalinks, Disable permalinks. It also means the same things.

Here, assume to place in functions.php

add_action( 'init', 'q363603_add_custom_rewrites' );
function q363603_add_custom_rewrites() {
        // take numbers to the query tl=numbers
        add_rewrite_rule('([0-9])+/?$', 'index.php?tl=$matches[1]', 'top');
}

add_filter('query_vars', 'q363603_add_custom_queryvars' );
function q363603_add_custom_queryvars( $qvars ) {
    // ask WordPress to allow this query parameter
    $qvars[] = 'tl';
    return $qvars;
}

edited: add permalinks turning on/off explanation