include($params['filename']);

1.Use a User Defined Tag.
2.Edit your config.php, change $config['use_smarty_php_tags'] from false to true and clear cache. Now you can insert php source with mask {php}your_source{/php}. Note: It's a bit risky and unsecure.
3. This is a mix of both. Create a User Defined Tag named extfile and insert the following

include($params['filename']);

CMSMS How to add php code in a page

ActionScript