How to add subdomain to htaccess

In your plugin:

<?php
register_activation_hook(WP_PLUGIN_DIR.'/your/plugin/path/plugin.php', 'wpse_set_subdomain');
function wpse_set_subdomain() {
    $siteurl = site_url();
    $subdomain = substr($siteurl, 8, 3); // may need to adjust which substring to grab
    update_option('wpse_subdomain', $subdomain);
}
?>

Then wherever you have conditional content:

<?php
$subdomain = get_option('wpse_subdomain');
if($subdomain == 'www') {
    // www code
} else {
    // other code
}
?>

As mentioned in the code comment, you may need to tweak which substring to grab by changing the numbers 8 and 3 (they’ll grab the first 3 letters starting at character 8 in the siteurl).