Permalink settings are available in the WordPress admin settings. Sometimes you may not have the access to WordPress admin and you want to set permalinks settings from functions.php.
Instructions: Add the following code to the functions.php file of your WordPress theme.
The .htaccess mod_rewrite it is still necessary.
<?php
// set permalink
function set_permalink(){
global $wp_rewrite;
$wp_rewrite->set_permalink_structure('/%year%/%monthnum%/%postname%/');
}
add_action('init', 'set_permalink');
?>
You may also enjoy flushing permalinks once per hour cron job.
Is this function also working in a multisite install?