PHP XML parsing + daily authentication request
You could write a module where you implement hook_cron() to get the authentication ID and then store in the drupal DB (e.g. as variable via variable_set()). After that you can use it inside your templates without any worry. But you have to trigger your cron.php once a day via a sytem cron-job or the poormanscron module. For PHP xml-parsing take a look at SimpleXML.

EDIT: Module creation:

For this job it would be a simple module. Take a look at the module-dev-guide. First you create a YOURMODULE.info file. After that you put your code inside a YOURMODULE.module file. Than implement that hook_cron() like that:

function YOURMODULE_cron() {
     update_news_service_id();
}

function update_news_service_id() {
    // add your code here
}


One more comment: If you try to solve it this way you must consider calling update_news_service_id() from the template (or ..._preprocess()) if you run into an expired service id-error.