Help Code Review - I need to write on .htaccess file from theme's function.php

Question

I am trying to add support for Vary: Accept-Encoding from whithin my theme’s function.php file.

Please don’t suggest me to use any plugin, because that the only reason why I am trying to implement this from within my theme.

The idea of using insert_with_markers from this StackOverflow discussion: How to add .htaccess code through a function?

Can you please tell me what I have done wrong?

add_action('after_setup_theme','add_htaccess');
function add_htaccess($insertion)
    {
    $insertion = array(
        '# Enable Vary: Accept-Encoding Start',
        '<IfModule mod_headers.c>',
        '<FilesMatch "\.(js|css|xml|gz)$">',
        'Header append Vary: Accept-Encoding',
        '</FilesMatch>',
        '</IfModule>',
    );
    $htaccess_file = ABSPATH.'.htaccess';
    return insert_with_markers($htaccess_file, '# Testing', (array) $insertion);
}

Answer 1

The code below has work for me in the past :)

function add_to_htaccess( $rules ) {

  $content = <<<EOD
  # Enable Vary: Accept-Encoding Start
  <IfModule mod_headers.c>
  <FilesMatch "\.(js|css|xml|gz)$">
  Header append Vary: Accept-Encoding
  </FilesMatch>
  </IfModule>
  EOD;
 return $content . $rules;

}
add_filter('mod_rewrite_rules', 'add_to_htaccess');