Going through rewrite_rules_array seems unneeded - it is more for a rewrite rule kill/remove than for adding rules...

USING functions.php

Using WordPress theme functions (functions.php), there are 2 steps to pass slash-based url as GET parameters to a page, after any rewrite-rule change: Flush rewrite rules == re-save permalink settings in admin!

Step 1 - add_rewrite_rule(...)

This might be a bit confusing, when trying to get it work, because documentation for add_rewrite_rule(...) is not quite clear on where to target the rewrite nor how to target a page by slug... and there is just an important note that you need to flush rewrite rules every time you change something like that...

This works for me (WP 4.3.3)

add_action('init', function(){   add_rewrite_rule(       '^yourPageSlug/([^/]+)([/]?)(.*)',       //!IMPORTANT! THIS MUST BE IN SINGLE QUOTES!:      'index.php?pagename=yourPageSlug&user=$matches[1]',       'top'   );   });

Arguments/Parameters:

1. Regular expression ("regexp") to match the particular url structure
2. Rewrite template in which you should target index.php (if it is not an external link), because all url rewriting in WordPress comes through it
   • use pagename GET parameter set to your page slug
   • use placeholder $matches[indexStartingFrom1] for regexp match - which really
     is just a placeholder! ==> use single quotes to wrap this string!

3. 'top' or 'bottom'. 'top' will take precedence over WordPress's existing rules, where 'bottom' will check all other rules match first. Default: "bottom"

Step 2 - add query_vars

Step above is just a targeting, not passing the variables itself...

Since WordPress would clear the $_GET parameters "as are" and just throw them away (because it would not find anything suitable for it), we need to tell the WordPress that we want to use them by adding our custom query_vars.

We will still not be able to retrieve them through $_GET superglobal, however,
we will be able to get them using get_query_var(...)

We tell WordPress that we want to use those GET parameters by adding a query_vars filter

add_filter('query_vars', function( $vars ){    $vars[] = 'pagename';     $vars[] = 'user';     return $vars;});

Final step:
FLUSH REWRITE RULES == RESAVE PERMALINK SETTINGS!

Usage

Then to get value of GET param "user" use:

get_query_var( 'user' ) 

USING .htaccess

If you can, you can also use .htaccess rule (considering, /mysite/ is your rewrite base)

RewriteRule ^pagename/([^/]+)$  pagename?user=$1

Here's a thorough tutorial that gets close: http://www.rlmseo.com/blog/passing-get-query-string-parameters-in-wordpress-url/

In short, you define some new rewrite rules, and then hook them into WordPress via add_filter('rewrite_rules_array', 'my_rewrite_rules');

However, following that link's example, your resulting URL would be http://localhost/mysite/pagename/user/myname - note that /user/ (the name of your query variable) is still included.