Wordpress Rewrite Rule

Question

I have a standard page setup at http://mysite.com/mypage/

I'd like to setup a rewrite rule in my theme functions that takes any URLs such as:

http://mysite.com/mypage/helloworld/ <-- Currently shows "Not Found"

And translates it into:

http://mysite.com/mypage/?var=helloworld <-- Currently shows the standard page

NOTE ADDED:

"helloworld" is an unknown variable value, so I want ALL "/mypage/" urls to rewrite back, for example: http://mysite.com/mypage/abc/ http://mysite.com/mypage/1231241/ http://mysite.com/mypage/sdfasfdadsf/

Will all rewrite back to http://mysite.com/mypage/?var={path}

Answer 1

As the question was modified, I modify my answer. This task is relatively easy, doable just with a single rewrite rule, which converts everything after /mypage/ string into a variable - I call it var. As this is added to query_vars via hook, is reachable via global $wp_query; echo $wp_query->var;.

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

add_action( 'generate_rewrite_rules', 'binda_rewrite_rules' );
function binda_rewrite_rules( $wp_rewrite ) {           
   $mypage_id = 4; //setup your 'mypage' ID
    $wp_rewrite->rules = array(             
        'mypage/(.+?)/?$' => $wp_rewrite->index . '?page_id='.$mypage_id.'&var=' . $wp_rewrite->preg_index( 1 ),        
    ) + $wp_rewrite->rules;
}