Permalinks and custom get variables – the working way

For those who want to spare 5 hours of searching:

If you want your plugin to work like:
https://www.example_wp_page.com/yourplugin/custom_param/custom_variable

// this needs to run once somewhere in your setup:
add_action('init', 'myvar_flush_rewrite');

add_filter('query_vars', 'myvar_vars');
add_filter('rewrite_rules_array', 'custom_rewrite_rules');

function myvar_flush_rewrite() { global $wp_rewrite; $wp_rewrite->flush_rules(); }

function myvar_vars($public_query_vars) { $public_query_vars[] = 'custom_param';
return $public_query_vars; }

function custom_rewrite_rules($existing_rules) {
$new_rules = array('(yourplugin)/custom_param/([^/]+)/?$' => 'index.php?pagename=yourplugin&profil=$matches[2]');
$existing_rules=$new_rules + $existing_rules;
return $existing_rules;}

function some_plugin_function(){
echo get_query_var('custom_param');
// you'll get custom_variable here
}